Practical robust nonlinear PD controller for cable-driven parallel manipulators

This paper presents a model-free robust nonlinear PD (R-NPD) controller for cable-driven parallel manipulators (CDPMs) in joint space. Generally, in various mechanical manipulators and in particular CDPMs for fast and high-precision tracking, a precise dynamic model is required. However, the dynamic model of the robot is always contaminated with uncertainties such as nonlinear and time-varying parameters as well as external disturbances. For this purpose, in the proposed controller structure, the time-delay estimation (TDE) technique is used to indirectly use the robot dynamics into the control structure without need of its prior knowledge. Furthermore, a nonlinear PD controller is designed in joint space in such a way that the robot can track the reference trajectory quite fast and accurate, without the need for any auxiliary sensors. The stability of the closed-loop system has been examined through Lyapunov direct method, and it has been shown that tracking error remains uniformly ultimately bounded. Finally, to demonstrate the effectiveness of the proposed controller, simulations and experiments have been performed on two different categories of CDPMs, whose results show that the proposed control scheme outperforms modified TDE control method in practice.