Differential drive robot on ROS Indigo slipping

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Dear all,

For the last couple of days, I have been trying to create a URDF model of a simple (or that is what I thought) differential drive robot (libgazebo_ros_diff_drive and libgazebo_ros_joint_state_publisher plugins) with a laser sensor (libgazebo_ros_laser) mounted on it and simulate it using gazebo (using ROS indigo).

The laser readings are just fine, the problem comes when sending twist commands to the robot: it doesn't move. Activating the option View->Wireframe in gazebo's gui makes it easy to see that the wheels are actually spinning, but the robot does not move. It feels like it is stuck to the ground or slipping on ice.

I am sure that I am missing something, but I can't figure out what. After days of looking at other examples that work, I am totally lost. Any help would be greatly appreciated.

Here is the URDF file:

<?xml version="1.0"?>
<robot name="cleaning_robot">
<!--
<link name="base_footprint"/>
-->
<link name="base_link">
  <visual>
    <geometry>
      <cylinder length="0.25" radius="0.26"/>
    </geometry>
    <material name="gray">
      <color rgba="0.8 0.8 0.8 0.7"/>
    </material>
  </visual>
  <collision>
    <geometry>
      <cylinder length="0.3" radius="0.32"/>
    </geometry>
  </collision>
  <inertial>
      <origin xyz="0.01 0 0" rpy="0 0 0"/>
      <mass value="10.0"/>
      <inertia
        ixx="0.074" ixy="0.0" ixz="0.0"
        iyy="0.12" iyz="0.0"
        izz="0.15"/>
    </inertial>
</link>

<gazebo reference="base_link">
  <material value="Gazebo/White"/>
</gazebo>

<link name="left_wheel_link">
  <visual>
    <geometry>
      <cylinder length="0.04" radius="0.145"/>
    </geometry>
    <origin xyz="0 0 0" rpy="1.57075 0 0"/>
    <material name="blue">
      <color rgba="0 0 .8 1"/>
    </material>
  </visual>
  <collision>
    <geometry>
      <cylinder length="0.04" radius="0.145"/>
    </geometry>
    <origin xyz="0 0 0" rpy="1.57075 0 0"/>
  </collision>
  <inertial>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <mass value="0.01"/>
      <inertia
        ixx="0.01" ixy="0.0" ixz="0.0"
        iyy="0.01" iyz="0.0"
        izz="0.01"/>
    </inertial>
</link>

<gazebo reference="left_wheel_link">
  <mu1>1.0</mu1>
  <mu2>1.0</mu2>
  <kp>1000000.0</kp> 
  <kd>100.0</kd>
  <minDepth>0.001</minDepth>
  <maxVel>1.0</maxVel>
</gazebo>

<joint name="base_to_left_wheel" type="continuous">
  <parent link="base_link"/>
  <child link="left_wheel_link"/>
  <origin xyz="0 0.3 0" rpy="0 0 0"/>
  <axis xyz="0 1 0"/>
</joint>

<link name="right_wheel_link">
  <visual>
    <geometry>
      <cylinder length="0.04" radius="0.145"/>
    </geometry>
    <origin xyz="0 0 0" rpy="1.57075 0 0"/>
    <material name="blue"/>
  </visual>
  <collision>
    <geometry>
      <cylinder length="0.04" radius="0.145"/>
    </geometry>
    <origin xyz="0 0 0" rpy="1.57075 0 0"/>
  </collision>
  <inertial>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <mass value="0.01"/>
      <inertia
        ixx="0.01" ixy="0.0" ixz="0.0"
        iyy="0.01" iyz="0.0"
        izz="0.01"/>
    </inertial>
</link>
<gazebo reference="right_wheel_link">
  <mu1>1.0</mu1>
  <mu2>1.0</mu2>
  <kp>1000000.0</kp>
  <kd>100.0</kd>
  <minDepth>0.001</minDepth>
  <maxVel>1.0</maxVel>
</gazebo>


<joint name="base_to_right_wheel" type ...