[Gazebo 9] Could not find parameter robot_description on parameter server

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Hi, I know this question is asked a couple of times before, but I just couldn't find the right answer for me. So I'm trying to launch my URDF model with gazebo plug in and use ros_control package to control the movement. But as soon as rover_control.launch run, the error (Could not find parameter robot_description on parameter server) appears. Now, I think this happens because I messed up something in my launch file, my yaml and my urdf. So I will quote three of them here. Thank you for your patient, this is my first week of learning ros.

Control launch file:

<launch>

  <!-- Load joint controller configurations from YAML file to parameter server -->
  <rosparam file="$(find rover_project)/rover_description/config/rover_control.yaml" command="load"/>

  <!-- load the controllers -->
  <node name="controller_spawner" pkg="controller_manager" type="spawner" respawn="false"
    output="screen" ns="/rover" args="left_wheel_holder_to_left_wheel right_wheel_holder_to_left_wheel joint_state_controller"/>

  <!-- convert joint states to TF transforms for rviz, etc -->
  <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"
    respawn="false" output="screen">
    <remap from="/joint_states" to="/rover/joint_states" />
  </node>

</launch>

YAML file:

rover:
  # Publish all joint states -----------------------------------
  joint_state_controller:
    type: joint_state_controller/JointStateController
    publish_rate: 50  

  # Position Controllers ---------------------------------------
  left_wheel_holder_to_left_wheel:
    type: effort_controllers/JointPositionController
    joint: left_wheel_holder_to_left_wheel
    pid: {p: 100.0, i: 0.01, d: 10.0}
  right_wheel_holder_to_left_wheel:
    type: effort_controllers/JointPositionController
    joint: right_wheel_holder_to_left_wheel
    pid: {p: 100.0, i: 0.01, d: 10.0}

And the URDF file:

<?xml version="1.0"?>

<robot name="Rover Platform">

  <material name="blue">
    <color rgba="0 0 0.8 1"/>
  </material>

  <material name="grey">
    <color rgba="0.2 0.2 0 1"/>
  </material>    

  <link name="base_link">
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <box size="1.0 1.0 0.2"/>
      </geometry>
      <material name="blue"/>

    </visual>

    <collision>
      <geometry>
        <box size="1.0 1.0 0.2"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="5"/>
      <inertia ixx="0.83333" ixy="0.0" ixz="0.0" iyy="0.433333" iyz="0.0" izz="0.433333"/>
    </inertial>

  </link>

  <link name="right_wheel_holder_link">

    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <box size="0.3 0.2 0.2"/>
      </geometry>
      <material name="blue"/>
    </visual>

    <collision>
      <geometry>
        <box size="0.3 0.2 0.2"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="1"/>
      <inertia ixx="0.011" ixy="0.0" ixz="0.0" iyy="0.0067" iyz="0.0" izz="0.011"/>
    </inertial>

  </link>

  <link name="left_wheel_holder_link">

    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <box size="0.3 0.2 0.2"/>
      </geometry>
      <material name="blue"/>
    </visual>

    <collision>
      <geometry>
        <box size="0.3 0.2 0.2"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="1"/>
      <inertia ixx="0.011" ixy="0.0" ixz="0.0" iyy="0.0067" iyz="0.0 ...