Flexible performance-based robust control for a class of nonlinear systems with input saturation

This paper investigates the tracking control problem of multi-input and multi-output nonlinear systems simultaneously considering the user-specified tracking performance, the asymmetric input saturation, and external disturbances. By introducing the modification signals into the user-specified performance functions, we propose a flexible performance-based control scheme, featuring the capability of avoiding the violation of the resulted modified performance functions (MPFs) due to the input saturation. These modification signals are associated with input saturation errors (the errors between the actual and desired control inputs) and generated by a novel auxiliary system in which all the states are guaranteed to be nonnegative. Consequently, the MPF can degrade the user-specified tracking performance when saturation happens and can recover it when no saturation occurs. Furthermore, the disturbance observer is utilized to estimate external disturbances and relieve the burden on the selection of control gains. With the developed control scheme, it is theoretically shown that the output can always track the reference signal and satisfy the constraints of the MPFs. Finally, the numerical simulation results are presented to show the effectiveness of the proposed control scheme.