-------------------------------------------------------------------------------
   Right-hand-side of a nonlinear spring dynamics model.
-------------------------------------------------------------------------------
   Form:
   dX = NonlinearSpring( x, t, d )
-------------------------------------------------------------------------------

   ------
   Inputs
   ------
   x		(4,1)  State
   t       (1,1)  Time
   d       (1,1)  Input data structure

   -------
   Outputs
   -------
   dX  	(4,1)  State derivative

-------------------------------------------------------------------------------