Driving a simple robot model (1.5 Building a Robot with SDF Tutorial)

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Hello,

I'm trying to follow the tutorial that describes how to create a robot model from model.sdf file on Gazebo 1.5. I intend to make my own robot, and be able to drive it, differential or skid steering, doesn't matter. But I think there is something wrong with the robot provided, I'm not able to drive it by subscribing to /gazebo/cmd[underscore]vel topic, or using the differential drive plugin (for /gz/cmd[underscore]vel/ topic). I've narrowed the problems down and now I think the .sdf file provided has some problems (maybe a missing tag, link, or dummy container, or some wrong parameters). Since I don't know how exactly a sdf file is constructed and described, I need some help revising the model.sdf file below:

<?xml version="1.0" ?>
<sdf version="1.4">
  <model name="myrobot">
   <static>false</static>
    <link name="chassis">
      <pose>0 0 .1 0 0 0</pose>
    <inertial>
      <mass>1.0</mass>
      <inertia>
        <ixx>1.0</ixx>
        <ixy>0.0</ixy>
        <ixz>0.0</ixz>
        <iyy>1.0</iyy>
        <iyz>0.0</iyz>
        <izz>1.0</izz>
      </inertia>
    </inertial>
      <self_collide>1</self_collide>
      <collision name="collision">
        <geometry>
          <box>
            <size>.4 .2 .1</size>
          </box>
        </geometry>
      </collision>
      <visual name="visual">
        <geometry>
          <box>
            <size>.4 .2 .1</size>
          </box>
        </geometry>
      </visual>
      <collision name="caster_collision">
        <pose>-0.15 0 -0.05 0 0 0</pose>
        <geometry>
          <sphere>
            <radius>.05</radius>
          </sphere>
        </geometry>
        <surface>
          <friction>
            <ode>
              <mu>0</mu>
              <mu2>0</mu2>
              <slip1>1.0</slip1>
              <slip2>1.0</slip2>
            </ode>
          </friction>
        </surface>
      </collision>
      <visual name="caster_visual">
        <pose>-0.15 0 -0.05 0 0 0</pose>
        <geometry>
          <sphere>
            <radius>.05</radius>
          </sphere>
        </geometry>
      </visual>
    </link>
    <link name="left_wheel">
      <pose>0.1 0.13 0.1 0 1.5707 1.5707</pose>
      <collision name="collision">
        <geometry>
          <cylinder>
            <radius>.1</radius>
            <length>.05</length>
          </cylinder>
        </geometry>
      </collision>
      <visual name="visual">
        <geometry>
          <cylinder>
            <radius>.1</radius>
            <length>.05</length>
          </cylinder>
        </geometry>
      </visual>
    </link>
    <link name="right_wheel">
      <pose>0.1 -0.13 0.1 0 1.5707 1.5707</pose>
      <collision name="collision">
        <geometry>
          <cylinder>
            <radius>.1</radius>
            <length>.05</length>
          </cylinder>
        </geometry>
      </collision>
      <visual name="visual">
        <geometry>
          <cylinder>
            <radius>.1</radius>
            <length>.05</length>
          </cylinder>
        </geometry>
      </visual>
    </link>
    <joint type="revolute" name="left_wheel_hinge">
      <pose>0 0 -0.03 0 0 0</pose>
      <child>left_wheel</child>
      <parent>chassis</parent>
      <axis>
        <xyz>0 1 0</xyz>
      </axis>
    </joint>
    <joint type="revolute" name="right_wheel_hinge">
      <pose>0 0 0.03 0 0 0</pose>
      <child>right_wheel</child>
      <parent>chassis</parent>
      <axis>
        <xyz>0 1 0</xyz>
      </axis>
    </joint>


<!-- Sensor 3 -->
<include>
  <uri>model://hokuyo</uri>
  <pose>-0.2 0 0.2 0 0 15.75</pose>
</include>

<joint name="hokuyo_joint" type="revolute">
  <child>hokuyo::link</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 0 -1</xyz>
    <limit>
      <upper>0</upper>
      <lower>0</lower>
    </limit>
  </axis>
</joint>
<!--  -->


<!-- Sensor1 -->
<include>
  <uri>model://hokuyo2</uri>
  <pose>-0.2 0.04 0.2 0 0 15.15</pose>
</include>
<joint name="hokuyo_joint2" type ...