主讲人：Professor GIUSEPPE REGA

Department of Structural and Geotechnical Engineering, Sapienza University of Rome, Italy

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主持人：王连华副教授

时间：2016年10月26日星期三，9:00-11:00

地点：湖南大学土木工程学院风工程中心会议室

Abstract:

Analyzing the nonlinear dynamical behavior of a mechanical system entails investigating the stability of its response under variation of both controllable parameters and initial conditions, the latteraccounting for possible imperfections to be taken into account for safe system operations, which depend not only on local stability but also on the global dynamics associated with uncorrupted basinsof attraction surrounding each solution. This is certainly true for the popular micro/nano mechanical systems, where slight changes of the initial position/velocity at the nanoscale level can produce dramatic modifications of the overall dynamics. Among them, Atomic Force Microscopes (AFMs) working in noncontact mode can undergo the unwanted “jump-to-contact” phenomenon, or escape (in dynamical systems terms), due to the atomic attraction between the cantilever tip and the sample to be scanned, which can become stronger than the beam restoring elastic force,thus making the equilibrium configuration unstable and producing contacts between tip and sample responsible for errors in the topography process.

This lecture overviews a basket of meaningful results recentlyobtained in the field by the author and his collaborator.

1. First, the nonlinear dynamical behavior of a single-mode model of noncontact AFM under scan (parametric) and vertical (external) excitations is systematically analyzed,to identify the overall stability boundary in the excitation parameter space.

2. Then, an external feedback control aimed at keeping the system response to an operationally suitable one is inserted in a nonlinear continuum formulation of the noncontact AFM, and the weakly nonlinear dynamics is investigated via the multiple-scale method, highlighting the effectiveness of the control technique and enabling the derivation of a comprehensive system bifurcation structure.

3. Next, an extended parametric analysis of the response in the strongly non-linearregime allows to investigatetheeffects of the feedback controlthesystembehavior, through a variety of bifurcation diagrams and response charts. Meaningful criticalities of the control actuation as regards the dynamical integrity of the periodic bounded solutions and of the relevant basinsare highlighted, showing the possibly dangerous effects of a locally tailored control technique on the overall response scenario.

4. Thus, the last step consists of implementing a suitable alternative control procedure able to exploit just the system global dynamics, either analytically or numerically, to favorable affect the reduction and delay of the basin erosion phenomenon which leads to the unwanted jump-to contact.

This ends up to a more robust system operation which is definitely of major interest for safe engineering design.

Biography:

• Professor, Sapienza University of Rome, Italy

• Past President AIMETA (Italian Association of Theoretical and Applied Mechanics)

• Chairman, EUROMECH Nonlinear Oscillations Conference Committee

• Former Chairman, Ph.D. Program Structural and Geotechnical Engineering, Sapienza

• Former Chairman, Doctoral School in Civil Engineering and Architecture, Sapienza

• Former Head, Department of Structure, Water and Soil Engineering, University of L'Aquila

• Former President, Committee of Italian Professors of Solid and Structural Mechanics

• Italian Representative, IUTAM General Assembly

• Past Editor-in-Chief, Meccanica

• Past/present Associate Editor or Editorial/Advisory Board Member of several Archival Journals

• Many EUROMECH, IUTAM and other Conferences/Symposia organized

• More than 130 papers in 48 Archival Journals+65 Book and Edited Volume Chapters + nearly 100 Refereed Conference Proceedings Papers

• Main Editorship: 3 Books, 7 Special Issues in Archival Journals

• Current research interests: Nonlinear oscillations in applied mechanics and structural dynamics; Bifurcation and chaos in nonlinear dynamics; Control of oscillations and chaos; Reduced-order modelling in solid and structural mechanics; Global dynamics and integrity; Wave propagation; Smart materials; Micro- and nano-mechanics; Thermomechanical problems.