Adaptive fractional order non-singular terminal sliding mode controller for underwater soft crawling robots with parameter uncertainties and unknown disturbances

Unknown disturbances and nonlinear properties of materials bring challenges to the precise trajectory tracking control of underwater soft robots. An underwater soft crawling robot with high adaptability and mobility in a disturbed underwater environment is proposed. A dynamic model that can characterize asymmetric hysteresis, creep and rate-dependent hysteresis behaviours is presented to solve the problem of electromechanical nonlinear coupling and viscoelasticity affecting the modelling accuracy. Moreover, an adaptive fractional order non-singular terminal sliding mode trajectory tracking controller is proposed to eliminate the influence of underwater uncertain disturbance. The adaptive trajectory tracking controller can achieve fast switching gain and avoid over-tuning, which can effectively improve the accuracy and robustness of the controller. Finally, the stability of the controller is verified according to Lyapunov theory, and the superiority and effectiveness of the control method proposed are proven by numerical simulation and experiment.