  This paper proposes a sliding mode controller with smooth control effort for
a class of nonlinear plants. The proposed controller is created by allowing
some constant parameters of the earlier smooth sliding control (SSC) to vary as
a function of the output tracking error, improving the control chattering
alleviation in practical implementations. Furthermore, during the sliding mode,
the new scheme can synthesize a range of controllers, such as fixed gain PI
controllers and approximations of the standard Super-Twisting Algorithm (STA),
as well as, the variable gain Super-Twisting Algorithm (VGSTA). A complete
closed-loop stability analysis is provided. In addition, realistic simulation
results with an unmanned aerial vehicle (UAV) model, incorporating aerodynamic
effects and internal closed-loop controllers, are obtained and validated via
experiments with a commercial hexacopter.
