Model of a smooth thooth signal.
  
   
  
  The output ( y ) remains at its initial value yin until time 
  is equal to ts.
  Then the signal changes smoothly. After the smoth start the signal grows up linearly with a slope m.
  When the output signal is close to the final value yfin, it is reached smoothly.
  At time td the signal returns at its initial value in a specular way.
  The parameter alfa (that has to be coprises between 0 and 1), indicates the portion of the rising 
  time occupied by the smooth junction.
  In figure are compared two models that have respectively alfa = 0.1 and alfa = 0.4.
  
Fixed Point not available in this model