the Department of Electrical and Computer Engineering University of Singapore 119077;
the School of Electrical and Electronic Engineering Technological University 639798;
the State Key Laboratory of Power Transmission Equipment&System Security and New Technology Key Laboratory of Intelligent Unmanned Systems of Automation University 400044;
the UTA Research Institute of Texas at Arlington Worth 76118 USA;
Adaptive control; enhanced Nussbaum function(ENF); strict-feedback systems; unified prescribed performance;