Creating a Constant Force Spring

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Have you tried using the SDF - Joint - Stiffness Element?

Example:

<joint name="gas_spring_joint" type="prismatic">
  <parent>cylinder</parent>
  <child>piston</child>
  <pose>0 0 0 0 0 0</pose>
  <axis>
    <limit>
      <lower>-0.1</lower>
      <upper>0.1</upper>
    </limit>
    <xyz>0.0 0.0 1.0</xyz>
    <dynamics>
      <spring_stiffness>2500</spring_stiffness>
      <spring_reference>0.06</spring_reference>
      <damping>20.0</damping>
      <friction>0.75</friction>
    </dynamics>
    <use_parent_model_frame>0</use_parent_model_frame>
  </axis>
  <physics>
    <ode>
      <implicit_spring_damper>1</implicit_spring_damper>
      <cfm_damping>1</cfm_damping>
      <limit>
        <cfm>0</cfm>
        <erp>0.2</erp>
      </limit>
    </ode>
  </physics>
</joint>

Also, see http://answers.gazebosim.org/question...