  This paper proposes a novel observer-based disturbance estimation method for
high degree-of-freedom Euler-Lagrangian systems using an unknown input-output
(UIO) sliding mode observer (SMO). Different from the previous SMO methods,
this approach does not assume the matching condition of the disturbances.
Besides, compared to the conventional disturbance estimation methods, the
proposed method does not require the calculation of the inverse inertia
matrices and accurate measurement of system velocities. This advantage resolves
the concerns of heavy computational load and amplified noise for practical
problems like external torque estimation of high degree-of-freedom manipulators
in safe human-robot collaboration. We achieve this by defining a novel
linearized model for the Euler-Lagrangian system and designing a
sliding-mode-based disturbance observer. The estimation precision of the
observer is ensured by the Lyapunov-based stability proof and the equivalent
control theory of sliding mode. At the end of this paper, the method is
implemented on a seven-degree-of-freedom robot manipulator, and the
experimental results confirm its decent performance and potential to be applied
in practice.
