[ros-users] Urdf joints/links question

Adolfo Rodríguez Tsouroukdissian adolfo.rodriguez at pal-robotics.com
Tue Jun 1 13:48:40 UTC 2010


On Tue, Jun 1, 2010 at 3:32 PM, Kirila Adamova <kirila.adamova at dfki.de>wrote:

> Hi all,
>
> I am currently writing a 3D model in urdf. I have two cylinders, which
> are connected with a joint. The second cylinder, however, is rotated and
> can be rotated further in the same direction. When I visualize them in
> gazebo, the joint is in the center of the second cylinder. Is there a
> way to make it at the end of the cylinder because that is the point
> around which I need to rotate it. To make it more clear (or even more
> confusing), here is sample code:
>
> <link name="feet">
>        <inertial>
>            <origin xyz="0 0 0.275" rpy="0 0 0"/>
>            <mass value="100"/>
>            <inertia ixx="1" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
>        </inertial>
>        <visual>
>            <origin xyz="0 0 0" rpy="0 0 0"/>
>            <geometry>
>                <cylinder radius="0.100" length="0.250"/>
>            </geometry>
>        </visual>
>        <collision>
>                  <origin xyz="0 0 0" rpy="0 0 0"/>
>                  <geometry>
>                    <cylinder radius="0.100" length="0.230"/>
>                  </geometry>
>            </collision>
>    </link>
>
>    <link name="hips">
>        <inertial>
>            <origin xyz="0 0 0.570" rpy="0 2.2 0"/>
>            <mass value="50"/>
>            <inertia ixx="1" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
>        </inertial>
>        <visual>
>            <origin xyz="0 0 0" rpy="0 2.2 0"/>
>            <geometry>
>                <cylinder radius="0.060" length="0.420"/>
>            </geometry>
>        </visual>
>        <collision>
>                  <origin xyz="0 0 0" rpy="0 2.2 0"/>
>                  <geometry>
>                    <cylinder radius="0.060" length="0.400"/>
>                  </geometry>
>            </collision>
>    </link>
>
>    <joint name="knees" type="revolute">
>        <parent link="feet"/>
>        <child link="hips"/>
>        <origin xyz="0 0 0.275"/>
>        <axis xyz="0 0 1"/>
>        <limit lower="1.31" upper="3.22" effort="30" velocity="1.0"/>
>    </joint>
>

> Currently, the axis xyz is set to z, although it needs to be y, because
> when it's set somewhere else, the whole model goes crazy (jumps and
> turns around) in gazebo.
>


Kirila,

The cylinder primitive makes its center coincide with the corresponding
origin (visualization, collision) and is aligned with the z axis. For your
robot, it should suffice to specify an appropriate origin for your
visualization and collision geometries (with respect to the joint origin).
For more details refer to the available documentation:

http://www.ros.org/wiki/urdf/XML/Link
http://www.ros.org/wiki/urdf/XML/Joint

HTH,

Adolfo


> Help will be greatly appreciated,
>
> Kirila
>
> p.s. End result must rotate the second cylinder around the point between
> the two cylinders around the y axis.
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-- 
Adolfo Rodríguez Tsouroukdissian, Ph. D.

Robotics engineer
PAL ROBOTICS S.L
http://www.pal-robotics.com
Tel. +34.93.414.53.47
Fax.+34.93.209.11.09
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