I. Introduction
Dynamic vibration absorbers known as DVAs, are practical devices that can neutralize or reduce the effects of an external harmonic excitation applied to a vibrating system. Passive DVAs consists of simple mechanical constituents such as mass and spring or pendulum. These DVAs are applicable only to a small range about the resonance frequency of the main system. Adaptive Passive DVAs known as virtual absorbers have been developed to extend the ability of the absorber in neutralizing the oscillation of the main system. The characteristics of such virtual absorbers are programmed to be updated and adapted to the uncertainties or the changes of the main system parameters. This can be achieved by adjusting the flexible part of the virtual absorber. Shang- Teh Wu and Yi-Chih Chuang [1], developed a virtual vibration absorber attached to a two-link robot with revolving arm. Regulation to a specific point and suppression the vibration of the system are considered to be successfully performed by virtual absorber. In another article, Shang-Teh Wu and Ying-Jhe Shao [2], studied the effectiveness of an adaptive control algorithm, which is mathematically equivalent to a virtual mass-spring absorber, on suppressing the vibration of a flexible system under harmonic excitation with uncertain frequency. Yuvaraja Mani and Mouleeswaran Senthilkumar [3] by the help of shape memory alloy spring, developed an adaptive passive absorber to attenuate the vibration of cantilever system. Shang-Teh Wu, Yea-Ying Chiu and Yuan-