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CONTENTS
Volume 10, Number 3, September 2012
 

Abstract
New insights into our previously proposed hybrid-type method for vibration control are highlighted in terms of energy analysis, such as the assessment of energy efficiency and system stability. The hybrid method improves the bang-bang active method by combining it with an energy-recycling approach. Its simple configuration and low energy-consumption property are quite suitable especially for isolated structures whose energy sources are strictly limited. The harmful influence of the external voltage is assessed, as well as its beneficial performance. We show a new chattering prevention approach that both harvests electrical energy from piezoelectric actuators and eliminates the displacement-offset of the equilibrium point of structures. The amount of energy consumption of the hybrid system is assessed qualitatively and is compared with other control systems. Experiments and numerical simulations conducted on a 10-bay truss can provide a thorough energy-efficiency evaluation of the hybrid suppression system having our energy-harvesting system.

Key Words
hybrid-type vibration control; semi-active vibration suppression; piezoelectric; energy-transfer; switching control

Address
Kanjuro Makihara: Department of Aerospace Engineering, Tohoku University, 6-6-01 Aramaki-Aza-Aoba, Aoba-ward, Sendai 980-8579, Japan

Abstract
Structural health monitoring with wireless sensor networks has received much attention in recent years due to the ease of sensor installation and low deployment and maintenance costs. However, sensor network technology needs to solve numerous challenges in order to substitute conventional systems: large amounts of data, remote configuration of measurement parameters, on-site calibration of sensors and robust networking functionality for long-term deployments. We present a structural health monitoring network that addresses these challenges and is used in several deployments for monitoring of bridges and buildings. Our system supports a diverse set of sensors, a library of highly optimized processing algorithms and a lightweight solution to support a wide range of network runtime configurations. This allows flexible partitioning of the application between the sensor network and the backend software. We present an analysis of this partitioning and evaluate the performance of our system in three experimental network deployments on civil structures.

Key Words
structural health monitoring; wireless sensor networks; monitoring software; bridge monitoring; real-life deployment; TinyOS; cable stayed bridge

Address
Kallirroi Flouri, Khash Erdene Jalsan and Glauco Feltrin : Structural Engineering Laboratory, Empa Dubendorf, Switzerland
Olga Saukh: Computer Engineering and Networks Laboratory, ETH Zurich, Switzerland
Robert Sauter: Networked Embedded Systems Group, University of Duisburg-Essen, Germany
Reinhard Bischoff and Jonas Meyer : Decentlab GmbH, Ueberlandstrasse 129, CH-8600 Dubendorf, Switzerland

Abstract
Guided waves have shown a great potential for structural health monitoring (SHM) applications. In contrast to traditional non-destructive testing (NDT) methodologies, a key element of SHM approaches is the high process of automation. The monitoring system should decide autonomously whether the host structure is intact or not. A basic requirement for the realization of such a system is that the sensors are permanently installed on the host structure. Thus, baseline measurements become available that can be used for diagnostic purposes, i.e., damage detection, localization, etc. This paper contributes to guided wave-based inspection in anisotropic materials for SHM purposes. Therefore, computational strategies are described for both, the solution of the complex equations for wave propagation analysis in composite materials based on exact elasticity theory and the popular global matrix method, as well as the underlying equations of two active damage localization algorithms for anisotropic structures. The result of the global matrix method is an angular and frequency dependent wave velocity characteristic that is used subsequently in the localization procedures. Numerical simulations and experimental investigations through time-delay measurements are carried out in order to validate the proposed theoretical model. An exemplary case study including the calculation of dispersion curves and damage localization is conducted on an exemplary unidirectional composite structure where the ultrasonic signals processed in the localization step are simulated with the spectral element method. The proposed study demonstrates the capabilities of the proposed algorithms for accurate damage localization in anisotropic structures.

Key Words
structural health monitoring; damage localization; guided ultrasonic waves; global matrix method; dispersion analysis

Address
Jochen Moll :Goethe University of Frankfurt, Department of Physics, Terahertz Photonics Group, Max-von-Laue-Strasse 1, 60438 Frankfurt am Main, Germany
Miguel Angel Torres-Arredondo and Claus-Peter Fritzen : University of Siegen, Institute of Mechanics and Control Engineering – Mechatronics, Paul-Bonatz-Stra

Abstract
A split spectrum processing (SSP) method is proposed to accurately determine the time-of-flight (ToF) of damage-scattered waves by comparing the instantaneous amplitude variation degree (IAVD) of a wave signal captured from a damage case with that from the benchmark. The fundamental symmetrical (S0) mode in aluminum plates without and with a notch is assessed. The efficiency of the proposed SSP method and Hilbert transform in determining the ToF of damage-scattered S0 mode is evaluated for damage identification when the wave signals are severely contaminated by noise. Broadband noise can overwhelm damage-scattered wave signals in the time domain, and the Hilbert transform is only competent fordetermining the ToF of damage-scattered S0 mode in a noise-free condition. However, the calibrated IAVD of the captured wave signal is minimally affected by noise, and the proposed SSP method is capable of determining the ToF of damage-scattered S0 mode accurately even though the captured wave signal is severely contaminated by broadband noise, leading to the successful identification of damage (within an error on the order of the damage size) using a triangulation algorithm.

Key Words
split spectrum processing (SSP); instantaneous amplitude variation degree (IAVD); time of flight (ToF); guided waves; broadband noise; triangulation algorithm

Address
X.T. Miao, F.C. Li, X.W. Sun and Guang Meng : State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai 200240, China
Lin Ye and Ye Lu : Laboratory of Smart Materials and Structures (LSMS), Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia
H.K. Peng : Shanghai Institute of Satellite Engineering, Shanghai 200240, China

Abstract
Researchers have made significant progress in recent years towards realizing effective structural health monitoring (SHM) utilizing wireless smart sensor networks (WSSNs). These efforts have focused on improving the performance and robustness of such networks to achieve high quality data acquisition and distributed, in-network processing. One of the primary challenges still facing the use of smart sensors for longterm monitoring deployments is their limited power resources. Periodically accessing the sensor nodes to change batteries is not feasible or economical in many deployment cases. While energy harvesting techniques show promise for prolonging unattended network life, low power design and operation are still critically important. This research presents the WiSeMote: a new, fully integrated ultra-low power wireless smart sensor node and a flexible base station, both designed for long-term SHM deployments. The power consumption of the sensor nodes and base station has been minimized through careful hardware selection and the implementation of power-aware network software, without sacrificing flexibility and functionality.

Key Words
structural health monitoring (SHM); wireless smart sensors; mesh networks; ultra-low power

Address
Davis P. Hoover and Argenis Bilbao : Department of Electrical and Computer Engineering, Texas Tech University, Lubbock, Texas, USA
Jennifer A. Rice: Engineering School of Sustainable Infrastructure and Environment, University of Florida, Gainesville, Florida, USA

Abstract
The authors\' research efforts recently led to the development of a customized wireless control unit which receives the real-time feedbacks from the sensors, and elaborates the consequent control signal to drive the actuator(s). The controller is wireless in performing the data transmission task, i.e., it receives the signals from the sensors without the need of installing any analogue cable connection between them, but it is powered by wire. The actuator also needs to be powered by wire. In this framework, the design of a power management unit is of interest only for the wireless sensor stations, and it should be adaptable to different kind of sensor requirements in terms of voltage and power consumption. In the present paper, the power management efficiency is optimized by taking into consideration three different kinds of accelerometers, a load cell, and a non-contact laser displacement sensor. The required voltages are assumed to be provided by a power harvesting solution where the energy is stored into a capacitor.

Key Words
wireless sensor; structural control; power harvesting; power management; frequency division multiplexing

Address
Sara Casciati : Department DICA, University of Catania, piazza Federico di Svevia, 96100 Siracusa, Italy
Lucia Faravelli and Zhicong Chen : Department DICAR, University of Pavia, Via Ferrata 1, 27100 Pavia, Italy


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