How to simulate a Stewart Platform with Gazebo and ROS?

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Hi :)

For an own project I want to move a platform with a pneumatic system (like the Stewart platform and for this I need a simulation with Gazebo. I'm really a beginner in using Gazebo and ROS and so I started with some tests (and tutorials) to learn how to use both systems. To begin with a "simple urdf-file" I created a table (1 platform with 4 legs) where I want to change the size of the legs. My aim is that Gazebo simulates the position of the platform when I change the size of each leg. But now I have some problems where I don't know how to solve them - maybe you can help me :)

I'm using the standalone version of Gazebo with Groovy on Ubuntu 12.04 32-Bit.

1) Unfortunately the tutorials ends before the description of how to manipulate the robot with controllers (here) and in addition I can't find a controller which simulates a pneumatic actuator. So I decided to create two boxes and to change the position of the inner box with a c++-program - this works for the moment very nice :) The problem is that I have to switch of the gravity for the inner box to avoid that it falls down - do you have a better idea to solve this problem?

  <!-- Link 1 (outer box)-->
  <joint name="Pn1_joint1" type="fixed">
    <origin xyz="0 0 0"/>
    <parent link="world"/>
    <child link="Pn1_link1"/>
  </joint>

  <link name="Pn1_link1">
    <collision>
    <origin xyz="0 0 0.05" rpy="0 0 0"/>
      <geometry>
    <box size="${widthPneumatic} ${widthPneumatic} 0.1"/>
      </geometry>
    </collision>
    <visual>
      <origin xyz="0 0 ${heightPneumatic/2}" rpy="0 0 0"/>
      <geometry>
    <box size="${widthPneumatic} ${widthPneumatic} ${heightPneumatic}"/>
      </geometry>
      <material name="orange"/> 
    </visual>
    <inertial> 
      <origin xyz="0 0 ${heightPneumatic/2}" rpy="0 0 0"/> 
      <mass value="1"/>            
      <inertia
      ixx="1.0" ixy="0.0" ixz="0.0"
      iyy="1.0" iyz="0.0"
      izz="1.0"/>
    </inertial>
  </link>

  <gazebo reference="Pn1_link1">
    <material>Gazebo/Orange</material>
  </gazebo>          

  <!-- Link 2 (inner box) -->
  <joint name="Pn1_joint2" type="prismatic">
    <origin xyz="0 0 0"/>
    <parent link="Pn1_link1"/>
    <child link="Pn1_link2"/>
    <axis xyz="0 0 1"/>
    <dynamics damping="100"/>
    <limit lower="0" upper="${heightPneumatic}" effort="0.00001" velocity="0.00001"/>
  </joint>

  <link name="Pn1_link2">
    <collision>
      <origin xyz="0 0 ${heightPneumatic/2}" rpy="0 0 0"/>
      <geometry>
    <box size="${widthPneumatic-diffInOut} ${widthPneumatic-diffInOut} ${heightPneumatic+diffInOut}"/>
      </geometry>
    </collision>
    <visual>
      <origin xyz="0 0 ${heightPneumatic/2}" rpy="0 0 0"/>
      <geometry>
    <box size="${widthPneumatic-diffInOut} ${widthPneumatic-diffInOut} ${heightPneumatic+diffInOut}"/>
      </geometry>
      <material name="orange"/>
    </visual>
    <inertial>
      <origin xyz="0 0 ${heightPneumatic/2}" rpy="0 0 0"/>
      <mass value="1"/>
      <inertia
      ixx="1.0" ixy="0.0" ixz="0.0"
      iyy="1.0" iyz="0.0"
      izz="1.0"/>
    </inertial>
  </link>

  <gazebo reference="Pn1_link2">
    <material>Gazebo/Orange</material>
    <turnGravityOff>true</turnGravityOff>
    <mu1>100</mu1>
    <mu2>100</mu2>
    <selfCollide>true</selfCollide>
  </gazebo>

2) So I added a second one to the world and wanted to connect these two actuators with the platform. There the tree structure of the urdf-file causes ... (more)