[BUG] Terrible control on freight robot without casters

[marcelino-pensa]

Describe the bug
I've been struggling with commanding the freight robot in gazebo. I had noticed that the full fetch robot worked fine, so I started looking into the differences between them until I could determine the issue.
As it turns out, the freight robot just needed some casters to be controlled reasonably.

To Reproduce
To reproduce the error, build fetch_gazebo on branch gazebo11 for ROS Noetic on Ubuntu 20.04.

• On one terminal, launch the simulation:

roslaunch fetch_gazebo playground.launch robot:=freight

• On a second terminal, launch teleop:

rosrun teleop_twist_keyboard teleop_twist_keyboard.py

Request the robot to move forward by pressing i on the keyboard.

Expected behavior
The robot should go forward.

Screenshots

Robot without casters (it doesn't even move forward, it ends up curving over time and has lots of jitter):

Robot with casters (similar to the navigation of the full robot):

catkin workspace (please complete the following information):

• fetch_gazebo version: gazebo11
• Ubuntu version: 20.04
• ROS version: Noetic

Solution
Add the following to freight.gazebo.xacro:

  <xacro:macro name="caster" params="prefix joint_x joint_y">
  <link name="${prefix}_caster_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <sphere radius="0.0601"/> <!-- 0.06033 -->
      </geometry>
      <material name="">
        <color rgba="0.086 0.506 0.767 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <sphere radius="0.0601"/>
      </geometry>
    </collision>
    <inertial>
      <inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <mass value="0.01"/>
    </inertial>
  </link>
  <gazebo reference="${prefix}_caster_link">
    <mu1>0.1</mu1>
    <mu2>0.1</mu2>
    <kp>100000000</kp>
    <kd>10.0</kd>
    <maxVel>10</maxVel>
    <minDepth>0.001</minDepth>
  </gazebo>
  <joint name="${prefix}_caster_joint" type="fixed">
    <parent link="base_link"/>
    <child link="${prefix}_caster_link"/>
    <origin rpy="0 0 0" xyz="${joint_x} ${joint_y} 0.055325"/>
    <axis xyz="0 1 0"/>
  </joint>
  </xacro:macro>
  <xacro:caster prefix="fl" joint_x="0.15" joint_y="0.12"/>
  <xacro:caster prefix="fr" joint_x="0.15" joint_y="-0.12"/>
  <xacro:caster prefix="br" joint_x="-0.2" joint_y="0.12"/>
  <xacro:caster prefix="bl" joint_x="-0.2" joint_y="-0.12"/>