I. Introduction
Uncertain nonlinearity exists in a wide range of practical systems, such as biological systems [1], mechanical systems [2], synchronous generator system [3], and navigation systems [4]. Fuzzy-logic systems (FLSs) [5] and neural networks [6] are powerful tools to deal with uncertain nonlinearity in the cases of single integrators [7], double integrators [8], and high-order nonlinear systems [9]. However, most results in the line of research require the condition of known control direction, that is, the sign of the control coefficient is known.