Solving inverse kinematics for special configuration (IKFast)

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I'm trying to solve the inverse kinematics problem for a custom made robot arm. The robot arm has a special design with the following 5 DoF in serial configuration:

• x-axis prismatic joint
• y-axis prismatic joint
• revolute joint about z-axis
• revolute joint about x-axis
• z-axis prismatic joint

Therefore, I can not reach every configuration in space, namely I'm limited in one of the end-effector angles. For better understanding of the model, I added the URDF at the end.

I was trying to use IKFast to generate an inverse kinematics .cpp file. But when using IK Type "Transform6D" I get an error:

CannotSolveError: need 6 joints

I must admit that it's not totally clear for me what this IK Types mean. I know how to calculate kinematics and inverse kinematics by hand and I can not imagine why this IK Type should matter.

Even trying "TranslationDirection5D" did results in:

CannotSolveError: raghavan roth equations too complex

So I'm wondering if IKFast can solve my problem or how I can do it. I just need the inverse kinematics calculations as "stand alone" (for simplicity) which I will then use in my ROS node. I was following this tutorial:

http://wiki.ros.org/Industrial/Tutori...

URDF file (Link 7 is just a tool frame with fixed Joint 6):

<robot name="or_robot">
  <link name="link1" />
  <link name="link2" />
  <link name="link3" />
  <link name="link4" />
  <link name="link5" />
  <link name="link6" />
  <link name="link7" />

  <joint name="joint1" type="prismatic">
    <parent link="link1"/>
    <child link="link2"/>
    <origin xyz="0 0 0.2" rpy="0 0 0" />
    <axis xyz="0 1 0" />
    <limit effort="1000.0" lower="0.0" upper="0.05" velocity="0.5"/>
  </joint>

  <joint name="joint2" type="prismatic">
    <parent link="link2"/>
    <child link="link3"/>
    <origin xyz="0 0 0" rpy="0 0 0" />
    <axis xyz="1 0 0" />
    <limit effort="1000.0" lower="0.0" upper="0.05" velocity="0.5"/>
  </joint>

  <joint name="joint3" type="revolute">
    <parent link="link3"/>
    <child link="link4"/>
    <origin xyz="0 0 0" rpy="0 0 0" />
    <axis xyz="0 0 1" />
    <limit effort="1000.0" lower="-0.4" upper="1.57" velocity="0.5"/>
  </joint>

  <joint name="joint4" type="revolute">
    <parent link="link4"/>
    <child link="link5"/>
    <origin xyz="0.1 0 0" rpy="0 0.2618 0" />
    <axis xyz="0 1 0" />
    <limit effort="1000.0" lower="0.0" upper="0.785" velocity="0.5"/>
  </joint>

  <joint name="joint5" type="prismatic">
    <parent link="link5"/>
    <child link="link6"/>
    <origin xyz="0 0 -0.05" rpy="0 0 0" />
    <axis xyz="0 0 -1" />
    <limit effort="1000.0" lower="0.0" upper="0.048" velocity="0.5"/>
  </joint>

  <joint name="joint6" type="fixed">
    <parent link="link6"/>
    <child link="link7"/>
    <origin xyz="0 0 -0.02" rpy="0 0 0" />
  </joint>

</robot>

Edit: changed units