How to make specific link static?

Question

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">
  <parent link="world"/>
  <child link="base_link"/>
  <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>
</link>

<gazebo reference="base_link" >
  <material> Gazebo/Blue </material>
  <static>true</static>
</gazebo>

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.

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.

Answer 1

To add a static link, you add the the kinematic property to your urdf:

<gazebo reference="base_link" >
  <!-- other properties you want to define for the base link -->
  <kinematic> true </kinematic>
</gazebo>

Here is a list of the properties of links. There isn't a static property. There is a kinematic property. I found this source that said the following:

A kinematic link (as opposed to a dynamic link) is not affected by external forces. It can influence other bodies in its vicinity, but it's not influenced by them.

And i tested it and it worked perfectly.