# How to make specific link static?

I want to simulate a RR robot in gazebo and I want to place the first link over a box. This box shouldn't be a part of the simulation, and is there only for visual effect.

How can I make a specific link/object static?

I created a fixed joint with a link named "world". Also, I tried adding a static tag.

However, it still included in the physics calculations, resulting in a weird behaviour. It spins on its own and it tilts, like this:

Relevant parts of the urdf:

<link name="world"/>

<joint name="world_joint" type="fixed">
<origin rpy="0 0 0" xyz="0 0 ${base_height}"/> </joint> <link name="base_link"> <visual> ... </visual> <collision> ... </collision> <inertial> <mass value="100" /> <inertia ixx="0.1" ixy="0.0" ixz="0.0" iyy="0.1" iyz="0.0" izz="0.1" /> <origin rpy="0 0 0" xyz="0 0${-base_height/2}"/>
</inertial>

<material> Gazebo/Blue </material>
<static>true</static>
</gazebo>


<inertia ixx="100" ixy="0.0" ixz="0.0" iyy="100" iyz="0.0" izz="100" />


and now the base doesn't tilt and the link doesn't rotate on its own. However, as I specified it as a static link, I anticipated that it shouldn't have that behaviour no matter its inertia.

Edit After some more searching, i found out that you cannot specify a link as static (sdf documentation). However, i still expect that the fixed joint with the world frame would keep the box in place.

I also changed some parameters of the physics engine, namely erp and cfm (you can find them here), but the weird behaviour continued.

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• I made the inertia of the base_link greater <inertia ixx="100" ixy="0.0" ixz="0.0" iyy="100" iyz="0.0" izz="100" />  and now the base doesn't tilt and the link doesn't rotate on its own. However, as i specified it as a static link, i anticipated that it shouldn't have that behaviour no matter its inertia. Jan 22 at 18:51
• Have you tried removing inertia and collision settings altogether? If it's just a visual then try it with only the visuals. Jan 23 at 2:04
• I tried it, and now the gazebo gui doesn't load any model. I anticipated that, as the inertial information are required according to this Jan 23 at 9:23

<gazebo reference="base_link" >