Comments on "An exponentially stable adaptive friction compensator"

This note points out a technique error in the above paper (Liao and Chien, IEEE Trans. Automat. Contr., Vol. 45, p. 977-80, 2000). It is shown that without persistence of excitation (PE) of the system signals, the estimated Coulomb friction parameter cannot be guaranteed to converge to its true value. An analysis is provided for the PE condition of parameter asymptotic convergence.