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CONTENTS
Volume 5, Number 3, May 2009
 

Abstract
A control algorithm for seismic protection of building structures based on the theory of variable structural control or sliding mode control is presented. The paper focus in the design of sliding surface. A method for determining the sliding surface by pole assignment algorithm where the poles of the system in the sliding surface are obtained on-line, based on the frequency content of the incoming earthquake signal applied to the structure, is proposed. The proposed algorithm consists of the following steps: (i) On-line FFT process is applied to the incoming part of the signal and its frequency content is recognized. (ii) A transformation of the frequency content to the complex plane is performed and the desired location of poles of the controlled structure on the sliding surface is estimated. (iii) Based on the estimated poles the sliding surface is obtained. (iv) Then, the control force which will drive the response trajectory into the estimated sliding surface and force it to stay there all the subsequent time is obtained using Lyapunov stability theory. The above steps are repeated continuously for the entire duration of the incoming earthquake. The potential applications and the effectiveness of the improved control algorithm are demonstrated by numerical examples. The simulation results indicate that the response of a structure is reduced significantly compared to the response of the uncontrolled structure, while the required control demand is achievable.

Key Words
sliding mode control; FFT; pole assignment; earthquake engineering; structural dynamics

Address
Nikos G. Pnevmatikos and Charis J. Gantes; Metal Structures Laboratory, School of Civil Engineering, National Technical University of Athens, 9 Heroon Polytechneiou, GR-15780 Zografou, Greece

Abstract
As in any engineering application, the problem of structural assessment should face the different uncertainties present in real world. The main source of uncertainty in Health Monitoring System (HMS) applications are those related to the sensor accuracy, the theoretical models and the variability in structural parameters and applied loads. In present work, two methodologies have been developed to deal with these uncertainties in order to adopt reliable decisions related to the presence of damage. A simple example, a steel beam analysis, is considered in order to establish a liable comparison between them. Also, such methodologies are used with a developed structural assessment algorithm that consists in a direct and consistent comparison between sensor data and numerical model results, both affected by uncertainty. Such algorithm is applied to a simple concrete laboratory beam, tested till rupture, to show it feasibility and operational process. From these applications several conclusions are derived with a high value, regarding the final objective of the work, which is the implementation of this algorithm within a HMS, developed and applied into a prototype structure.

Key Words
uncertainty; Modal Interval Analysis (MIA); perturbation method; structural assessment; Health Monitoring System (HMS)

Address
Jose Campos e Matos; University of Porto, Faculty of Engineering, Rua Dr. Roberto Frias s/n, Civil Engineering Department, 4200-465 Porto, Portugal
Oscar Garcia; University of Girona, Campus Montilivi, Edificio P-IV, Automatic, Informatics and Electronic Department, 17071 Girona, Spain
Antpnio Abel Henriques; University of Porto, Faculty of Engineering, Rua Dr. Roberto Frias s/n, Civil Engineering Department, 4200-465 Porto, Portugal
Joan Ramon Casas; Technical University of Catalonia, School of Civil Engineering. C/Jordi Girona 1-3, Campus Norte, Edificio C1, 08034 Barcelona, Spain
Josep Vehi; University of Girona, Campus Montilivi, Edificio P-IV, Automatic, Informatics and Electronic Department, 17071 Girona, Spain

Abstract
Shape control of flexible structures using piezoelectric materials has attracted much attention due to its wide applications in controllable systems such as space and aeronautical engineering. The major work in the field is to find a best control voltage or an optimal placement of the piezoelectric actuators in order to actuate the structure shape as close as possible to the desired one. The current research focus on the investigation of static shape control of intelligent shells using spatially distributed piezoelectric curve beam actuators. The finite element formulation of the piezoelectric model is briefly described. The piezoelectric curve beam element is then integrated into a collocated host shell element by using nodal displacement constraint equations. The linear least square method (LLSM) is employed to get the optimum voltage distributions in the control system so that the desired structure shape can be well matched. Furthermore, to find the optimal placement of the piezoelectric curve beam actuators, a genetic algorithm (GA) is introduced in the computation model as well as the consideration of the different objective functions. Numerical results are given to demonstrate the validity of the theoretical model and numerical algorithm developed.

Key Words
shape control; piezoelectric curve beam; finite element method; voltage distribution; placement optimization

Address
Jian Wang, Guozhong Zhao and Hongwu Zhang; State Key Laboratory for Structure Analysis and Industrial Equipment, Department of Engineering Mechanics, Dalian University of Technology, Dalian 116024, P.R. China

Abstract
A fuzzy-sliding mode controller is presented to control the dynamics of semi-active suspension systems of vehicles using magneto-rheological (MR) fluid dampers. A full car model is used to design and evaluate the performance of the proposed semi-active controlled suspension system. Four mixed mode MR dampers are designed, manufactured, and integrated with four independent sliding mode controllers. The siding mode controller is designed to decrease the energy consumption and maintain robustness. In order to overcome the chattering of the sliding mode controllers, a fuzzy logic control strategy is merged into the sliding mode controller. The proposed fuzzy-sliding mode controller is designed and fabricated. The performance of the semi-active suspensions is evaluated in both the time and frequency domains. The obtained results demonstrate that the proposed fuzzy-sliding mode controller can effectively suppress the vibration of vehicles and improve their ride comfort and handling stability. Furthermore, it is shown that the

Key Words
MR fluid damper; semi-active suspension; fuzzy-sliding mode controller

Address
L. Zheng and Y. N. Li; State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing 400044, P.R. China
A. Baz; Department of Mechanical Engineering, University of Maryland, College Park, MD 20742, USA

Abstract
Two degrees of freedom resonant systems are employed to improve the resonant property of resonant sensor, as compared to a single degree of freedom resonant system. This paper presents design, development and testing of two degrees of freedom resonant sensor. To measure absolute mass, cantilever shaped two different masses (smaller/absorber mass and bigger/drive mass) with identical resonant frequency are mechanically linked to form 2 - Degree-of-Freedom (DOF) resonator which exhibits higher amplitude of displacement at the smaller mass. The same concept is extended for measuring differential quantity, by having two bigger mass and one smaller mass. The main features of this work are the 3 - DOF resonator for differential detection and the microcontroller based closed loop electronics for resonant sensor with piezoelectric sensing and excitation. The advantage of using microcontroller is that the method can be easily extended for any range of measurand.

Key Words
resonant sensor; 2 DOF resonator; piezoelectric and split mass

Address
G. Uma, M. Umapathy, K. Suneel Kumar, K. Suresh and A. Maria Josephine; Department of Instrumentation and Control Engineering, National Institute of Technology,
Tiruchirappalli-620 015, India

Abstract
Damage detection using structural classical deflection flexibility has received considerable attention due to the unique features of the flexibility in the last two decades. However, for relatively complex structures, most methods based on classical deflection flexibility fail to locate damage sites to the exact members. In this study, for structures whose members are dominated by axial forces, such as truss structures, a more feasible flexibility for damage detection is proposed, which is called the Axial Strain (AS) flexibility. It is synthesized from measured modal frequencies and axial strain mode shapes which are expressed in terms of translational mode shapes. A damage indicator based on AS flexibility is proposed. In addition, how to integrate the AS flexibility into the Damage Location Vector (DLV) approach (Bernal and Gunes 2004) to improve its performance of damage localization is presented. The methods based on AS flexbility localize multiple damages to the exact members and they are suitable for the cases where the baseline data of the intact structure is not available. The proposed methods are demonstrated by numerical simulations of a 14-bay planar truss and a five-story steel frame and experiments on a five-story steel frame.

Key Words
axial strain; flexibility; damage detection; damage localization

Address
Guirong Yan, Zhongdong Duan and Jinping Ou; School of Civil Engineering, Harbin Institute of Technology, Harbin, 150090, China


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