Position/Attitude Cascade Regulation of Nonholonomic Four-Wheeled Mobile Robot via Double-Loop Sliding-Mode Control Mechanism.

Nonholonomic four-wheeled mobile robot (NFMR) is a typical multiple input-multiple output system that formulates its kinematic dynamics concerning position and attitude in a parallel manner. However, due to the lumped disturbances and interconnected states, demand-satisfied performance is difficult to obtain for existing coupled control solutions. To address this problem, a double-loop sliding-mode control (DLSMC) mechanism is proposed for achieving position/attitude cascade regulation. For the outer position tracking loop in the proposed scheme, a sliding mode control method of the bounded time-varying integral nonsingular terminal is designed to guarantee fast tracking in the presence of large initial errors and input saturation. On the other hand, for the inner attitude control loop, a novel adaptive barrier function-based sliding-mode control method is proposed without control gain overestimation. This enables the attitude to follow within a predefined vicinity of the sliding mode surface and holds it subsequently independent of the lumped uncertainties. Theoretical analysis is conducted to demonstrate the asymptotic stability. Comparative experiments implemented on a homemade NFMR show enhanced trajectory tracking performance and system robustness using position/attitude cascade regulation via the proposed DLSMC mechanism.