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   Rotary friction model using differentiable functions. 
   This model uses functions that are differentiable making it suitable for 
   numerical simulations. 
   See also RWA.m and SmoothFrictionDemo.
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   Form:
   f = FrictionSmooth( x, d )
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   ------
   Inputs
   ------
   x			  (:)     rate
   d			  (:)     .fStatic   - Static friction maximum
                         .kStatic   - Static friction scaling coefficient
                         .fCoulomb  - Coulomb friction maximum
                         .kCoulomb  - Coulomb friction scaling coefficient
                                      (higher means a faster rise)
                         .bViscous  - Viscous friction coefficient

   -------
   Outputs
   -------
   f             (:)    Friction

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