I. Introduction
Adaptive control is an important branch of control field, which has been developed for several decades. Huge amount of valuable adaptive control schemes have been proposed, which are detailed introduced in [1]–[6] and the references therein. These adaptive control schemes are classified into "Lyapunov-based" and "estimation-based" (as known as "modular-based") approaches in [1]. The "Lyapunov-based" adaptive control schemes are designed to guarantee some desired properties of the Lyapunov functions, such as with α of a class K function. Although desired performance of the closed-loop systems and boundedness of the estimates are both guaranteed in theory, the effects of rapidly varying estimation and "differential explosion" problem of high-order systems are two disadvantages. These drawbacks bring difficulty to the control design in practical applications. "Estimation-based" adaptive control includes least-squares and gradient approaches, which are more effective in practical applications. However, the proof and analysis of "estimation-based" schemes are quite challenging, and it always depends on the persistently exciting (PE) conditions.