  Assurance of asymptotic trajectory tracking in robotic manipulators with a
smooth control law in the presence of unmodeled dynamics or external
disturbance is a challenging problem. Recently, it is asserted that it is
achieved via a rigorous proof by designing a traditional model-free controller
together with time delay estimation (TDE) such that neither dynamical
parameters nor conservative assumptions on external disturbance are required.
The purpose of this note is to show that this claim is not true and the
stability proof of the method is incorrect. Finally, some modified versions of
this controller with rigorous proof is presented for robotic manipulators.
