Introduction

Abstract

It was early 1970s, when researchers have observed that a large number of modern mechanical systems were using fewer control inputs (actuating inputs) than the number of output variables. Further inspections have revealed that those systems were exhibiting some of the common dynamical properties, and thereby could be categorized as a different subclass of the mechanical systems. Since those systems use fewer numbers of actuating inputs, they have been classified as the Underactuated mechanical systems (UMSs). A few examples of such systems are as follows different robotic systems, spacecrafts, underwater vehicles, surface vessel, helicopter, space robots, underactuated manipulators, etc. It has also been observed that this class of systems generates interesting control problems to which the naive design approaches of orthodox control theory could not be applicable. Inspired by their increasing demand in diverse industrial applications, control of the UMSs started to gain its popularity as one of the most active research fields among the control systems community.