Fractional robust finite time control of four-wheel-steering mobile robots subject to serious time-varying perturbations

The four-wheel-steering mobile robot (FMR) is widely applied in the manufacturing industry, where accurate and stable lateral motion control is a prerequisite for ensuring manufacturing quality and efficiency. However, serious time-varying perturbations such as system uncertainties and external disturbances usually lead to unsatisfactory control performance. By designing constrained prediction and sliding mode mechanisms, a novel adaptive fractional robust finite time controller is proposed to achieve a system with required control accuracy and stability under serious time-varying perturbations. Compared with existing FMR solutions, the proposed method has the following attractive properties: (1) Without requiring the derivatives of time-varying perturbations, the proposed method utilizes a modified fractional super-twisting sliding mode switching law to guarantee the system robustness of dynamical tracking and disturbance rejection; (2) The differences between the nominal predicted states and the feedback ones can be well accommodated despite unmodeled dynamics and external disturbance; (3) By designing continuous control inputs, the “chattering phenomenon” in conventional control laws is carefully handled. Moreover, sufficient conditions are derived for the variable control gains to ensure the input-to-state practical stability and finite time convergence. Under harsh working conditions, two comparative experiments implemented on a real-life FRM are performed for demonstrative purposes.