Is it possible to have both fixed and dynamic objects in Urdf Model? [closed]

asked 2012-08-21 02:08:31 -0500

johannesm gravatar image

Hi,

I want to create a simple urdf object consisting of a:

  • Box_A, which is fixed in the world. No matter what external forces apply on it, it should stay in the same place. (fixed)
  • Box_B, which is connect to Box_A with a revolute joint. (dynamic)

In " http://answers.ros.org/question/29437/possible-to-declare-static-object-in-urdf-file/ " a way to this was figured out using a world link:

<robot name="Test" xmlns:controller="..." xmlns:interface="..." xmlns:sensor="...">
<link name="world" />
<link name="Box_A">
 ... definition of the box
</link>
<link name="Box_B">
 ... definition of the box
</link>

<joint name="world_joint" type="fixed">
    <!-- Here I have to define the global position of Box_A -->
    <origin rpy="0 0 0" xyz="0 0 1"/>
    <parent link="world"/>
    <child link="Box_A"/>
</joint>
<joint name="boxa_to_boxb_joint" type="revolute">
    <axis xyz="1 0 0"/>
    <origin rpy="0 0 0" xyz="-0.15 0.1 0.00"/>
    <limit effort="0.01" lower="-3.14159265359" upper="1.57079632679" velocity="0.001"/>
    <parent link="Box_A"/>
    <child link="Box_B"/>
    <dynamics damping="1.0" friction="1.0"/>
    <safety_controller k_velocity="0.1"/>
</joint>
</robot>

This code works fine, resulting in a swinging Box connected to a fixed Box. However, this solution blocks the possibility to spawn the object at a certain position at runtime.

Can you think of other solutions which still use the spawn_urdf_model service's parameters (position, orientation)? For example, something like to use a static modifier on Box_A?

<gazebo reference="Box_A">
  <static>true</static>
</gazebo>

But that does unfortunately not work for me.

Thank you in advance, Johannes

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Closed for the following reason question is not relevant or outdated by tfoote
close date 2015-03-26 19:00:41.511575

Comments

1

Is it possible to write this as xacro then pass parameters in when launching?

mcevoyandy gravatar image mcevoyandy  ( 2012-08-21 03:40:56 -0500 )edit

Thanks for this idea. I have now written it as a xacro file. I include the needed height variables using environment variables. It is not very clean but it works.

Run in bash before: "export TEST_POS_X=4" In urdf.xacro included: "$(optenv TEST_POS_X 0)". Zero is default

johannesm gravatar image johannesm  ( 2012-08-24 04:31:45 -0500 )edit

not sure exactly what you mean, I was thinking more of the gazebo_worlds/table file more than anything else. Have you taken a look at that launch file? You should be able to do a similar thing and still launch your models from the command line...

mcevoyandy gravatar image mcevoyandy  ( 2012-08-24 05:51:16 -0500 )edit

The table launch files runs the spawn_model node. Unfortunately, if I use: "<link name="world" />" as my urdf's root node this makes the spawn_model node ignore all the position parameters i give it. The model will always be aligned to 0,0,0 then.

johannesm gravatar image johannesm  ( 2012-08-25 10:42:22 -0500 )edit

This would be more relevant on [Gazebo Answers](http://answers.gazebosim.org/questions/). URDF does not have the notion of static/moving objects as the virtual joint linking the object to the environment is (usually) not expressed. With SDF it would be different.

Thomas gravatar image Thomas  ( 2013-09-30 15:12:29 -0500 )edit