“…Since the control signals in (13) include the terms e i / e 2 and sgn(e i ) = |e i |/e i , i = 1, 2, 3, 4, the undesirable chattering phenomenon may occur. In order to remove the chattering, these terms are modified as e i /( e 2 + ε) and |e i |/(e i + ε), i = 1, 2, 3, 4, respectively, where ε > 0 is a sufficiently small constant [20,21].…”