Coulomb model is most adopted in engineering dry friction problems because of its simple expression.but this simple discontinuous nature causes big trouble in nonlinear analysis.Transfer features of stick-slip motion.which are essential to stability analysis of friction system, are hard to gain when a Coulomb model is used because transfer happens when relative velocity is small enough which is corresponding to the discontinuous area of Coulomb model.Different types of smoothening functions can be used to resolve this difficulty, and many papers have compared their performance in approximating the classic discontinuous nature and in describing friction characteristics [1-4].