Variable gain based composite trajectory tracking control for 4-wheel skid-steering mobile robots with unknown disturbances

This paper investigates the trajectory tracking problem for 4-wheel skid-steering mobile robots (SSMRs) with unknown disturbances. A novel variable gain based composite disturbance rejection control approach is proposed. By designing a variable gain disturbance observer, the unknown uncertainty and external disturbance are estimated. Then, a novel time-varying sliding surface and nonsingular terminal sliding mode controller (NTSMC) are proposed, and the estimation of disturbance is incorporated in the controller to achieve the active disturbance rejection. It is shown that, with the proposed disturbance observer and controller, the tracking error is guaranteed to converge within a predefined time. Moreover, the control allocation for motors is also considered, which can allocate the virtual driving force to obtain the motor driving torque commands for the four wheels. Finally, the stability and the tracking performance of the proposed control scheme are verified respectively by theoretical analysis and comparative experiments.