  The precise motion control of a multi-degree of freedom~(DOF) robot
manipulator is always challenging due to its nonlinear dynamics, disturbances,
and uncertainties. Because most manipulators are controlled by digital signals,
a novel higher-order sliding mode controller in the discrete-time form with
time delay estimation is proposed in this paper. The dynamic model of the
manipulator used in the design allows proper handling of nonlinearities,
uncertainties and disturbances involved in the problem. Specifically,
parametric uncertainties and disturbances are handled by the time delay
estimation and the nonlinearity of the manipulator is addressed by the feedback
structure of the controller. The combination of terminal sliding mode surface
and higher-order control scheme in the controller guarantees a fast response
with a small chattering amplitude. Moreover, the controller is designed with a
modified sliding mode surface and variable-gain structure, so that the
performance of the controller is further enhanced. We also analyze the
condition to guarantee the stability of the closed-loop system in this paper.
Finally, the simulation and experimental results prove that the proposed
control scheme has a precise performance in a robot manipulator system.
