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This study proposes a new sensor cluster configuration for localizing an acoustic source in a plate using uniform linear array beamforming and T-shaped sensor clusters. This technique requires neither the properties of the plate material nor a dense array of sensors to find the direction of arrival of the acoustic source. It functions by placing four sensors in a cluster in the shape of the letter "'T" over a small region of the plate. Uniform linear array beamforming-based source localization is carried out by the constructive interference of different sensor signals. However, this approach has the disadvantage of a very low resolution when the direction of arrival approaches certain values. The L-shaped sensor clusters use the information from the time difference of arrival between the sensors to estimate the direction of arrival, which has a high resolution in all directions except for the direction that is very close to vertical to the cluster. In this study, we numerically and experimentally demonstrate that the proposed T-shaped sensor cluster can accurately localize the acoustic source with no blind area. We also detail its superior performance compared to both uniform linear array beamforming and L-shaped clusters. In the experimental investigation, the maximum deviation of impact source localization was reduced significantly, from 54° to 4° for an aluminum plate, and from 42° to 3° for a composite plate. Furthermore, this novel combined sensor array layout requires only a few sensors, which can significantly reduce the cost of structural health monitoring practice. [ABSTRACT FROM AUTHOR]

This article presents the detection performance results for multiple detectors or test statistics, using different active-sensing hardware systems in identifying the presence and location of a through-thickness fatigue crack in a 9-m composite wind turbine rotor blade. The rotor blade underwent ~8.5 million cycles of fatigue loading until failure, when a 30-cm-long crack surfaced on the leading edge portion of the blade’s transitional root area. The rotor blade was cantilevered on a 7-ton test stand and excited using a hydraulically actuated resonant excitation system, which drove the rotor blade at its first natural frequency. Through the course of the test, data were collected using two distinct types of acquisition hardware: one designed for ultrasonic-guided wave interrogation and the other for diffuse wave field interrogation. This article presents the fatigue crack detection performance results for several hardware and test statistic combinations. [ABSTRACT FROM AUTHOR]

In this paper, time domain data from piezoelectric active-sensing techniques is utilized for structural health monitoring (SHM) applications. Piezoelectric transducers have been increasingly used in SHM because of their proven advantages. Especially, their ability to provide known repeatahle inputs for active-sensing approaches to SHM makes the devel-opment of SHM signal processing algorithms more efficient and less susceptible to opera-tional and environmental variability. However, to date, most of these techniques have been based on frequency domain analysis, such as impedance-based or high-frequency response functions-based SHM techniques. Even with Lamb wave propagations, most researchers adopt frequency domain or other analysis for damage-sensitive feature extraction. Therefore, this study investigates the use of a time-series predictive model which utilizes the data obtained fi'om piezoelectric active-sensors. In particular, time se-ries autoregressive models with exogenous inputs are implemented in order to extract damage-sensitive features from the measurements made by piezoelectric active-sensors. The test structure considered in this study is a composite plate, where several damage conditions were artificially imposed. The peiformance of this approach is compared to that of analysis based on frequency response functions and its capability for SHM is demonstrated. [ABSTRACT FROM AUTHOR]

This article presents a new signal-processing technique, which utilizes ‘‘relative baselines’’ instead of ‘‘pre-stored baselines,’’ for Lamb wave based SHM. Several successful SHM methods utilizing wave propagations usually involve recording baseline measurements and comparing them to a newly measured response for structural damage identification. However, maintaining an accurate database of baselines remains challenging because of the effects of varying environmental conditions. Therefore, in this study, the relative baseline concept is proposed, in which measured Lamb waves are correlated between different sensor-actuator sets, as opposed to being correlated to pre-stored baseline data. This study focuses on determining the feature best used for this relative baseline concept, and cross-correlation and power spectral density analysis techniques are performed on data sets recorded from composite and aluminum plates. Experiments are performed with these plates under the presence of temperature variations in order to demonstrate the capability of the relative baseline concept. Our experimental results clearly indicate that the proposed technique reduces the complications associated with using pre-stored baselines for SHM under varying environmental conditions, and provides a quantitative means of identifying structural damage. [ABSTRACT FROM AUTHOR]

This paper presents recent developments in an extremely compact, wireless impedance sensor node (the WID3, Wireless Impedance Device) for use in high-frequency impedance-based structural health monitoring (SHM), sensor diagnostics and validation, and low-frequency (< ∼1 kHz) vibration data acquisition. The WID3 is equipped with an impedance chip that can resolve measurements up to 100 kHz, a frequency range ideal for many SHM applications. An integrated set of multiplexers allows the end user to monitor seven piezoelectric sensors from a single sensor node. The WID3 combines on-board processing using a microcontroller, data storage using Hash memory, wireless communications capabilities, and a series of internal and external triggering options into a single package to realize a truly comprehensive, self-contained wireless active-sensor node for SHM applications. Furthermore, we recently extended the capability of this device by implementing low-frequency analog-to-digital and digital-to-analog converters so that the same device can measure structural vibration data. The compact sensor node collects relatively low-frequency acceleration measurements to estimate natural frequencies and operational deflection shapes, as well as relatively high-frequency impedance measurements to detect structural damage. Experimental results with application to SHM, sensor diagnostics and low-frequency vibration data acquisition are presented. [ABSTRACT FROM AUTHOR]

In this article, we present experimental investigations using energy harvesting and wireless energy transmission to power wireless structural health monitoring sensor nodes. The goal of this study is to develop sensing systems that can be permanently embedded within a host structure without the need for an on-board power source. With this approach the required energy will be harvested from the ambient environment, or periodically delivered by a radio-frequency energy source to supplement conventional harvesting approaches. This approach combines several transducer types to harvest energy from multiple sources, providing a more robust solution that does not rely on a single energy source. Both piezoelectric and thermoelectric transducers are considered as energy harvesters to extract the ambient energy commonly available on civil structures such as bridges. Methods of increasing the efficiency, energy storage medium, target applications and the integrated use of energy harvesting sources with wireless energy transmission will be presented. [ABSTRACT FROM AUTHOR]

This paper reviews the development of energy harvesting for low-power embedded structural health monitoring (SHM) sensing systems. A statistical pattern recognition paradigm for SHM is first presented and the concept of energy harvesting for embedded sensing systems is addressed with respect to the data acquisition portion of this paradigm. Next, various existing and emerging sensing modalities used for SHM and their respective power requirements are summarized followed by a discussion of SHM sensor network paradigms, power requirements for these networks, and power optimization strategies. Various approaches to energy harvesting and energy storage are discussed and limitations associated with the current technology are addressed. The paper concludes by defining some future research directions that are aimed at transitioning the concept of energy harvesting for embedded SHM sensing systems from laboratory research to field-deployed engineering prototypes. Finally, it is noted that many of the technologies discussed herein are applicable to powering any type of low-power embedded sensing system regardless of the application. [ABSTRACT FROM AUTHOR]

This paper illustrates an integrated approach for identifying structural damage. The method presented utilizes piezoelectric (PZT) materials to actuate/sense the dynamic response of the structures. Two damage identification techniques are integrated in this study, including impedance methods and Lamb wave propagations. The impedance method monitors the variations in structural mechanical impedance, which is coupled with the electrical impedance of the PZT patch. In Lamb wave propagations, one PZT patch acting as an actuator launches an elastic wave through the structure, and responses are measured by an array of PZT sensors. The changes in both wave attenuation and reflection are used to detect and locate the damage. Both the Lamb wave and impedance methods operate in high frequency ranges at which there are measurable changes in structural responses even for incipient damage such as small cracks, debonding, or loose connections. The combination of the local impedance method with the wave propagation based approach allows a better characterization of the system's structural integrity. The paper concludes with experimental results to demonstrate the feasibility of this integrated active sensing technology. [ABSTRACT FROM AUTHOR]

Self-sensing actuation allows a single piezoelectric (PZT) element to be simultaneously used as both a sensor and an actuator. A specially designed electric circuit, referred to as a bridge circuit, is required to realize the concept. However, precise equilibrium of the bridge circuit is extremely difficult to obtain because of the continuous changes in environmental conditions. In this study, the effects of an unbalanced bridge circuit are analytically and experimentally evaluated in an attempt to quantify the variations in the PZT capacitance in terms of performances in vibration testing and control. Once the dynamic characteristics of self-sensing actuation are identified and understood, methods for improving the system's performance are developed by utilizing capacitors in series and in parallel with the PZT patch. The analytical and experimental results clearly indicate that the new design scheme increases the stability of the system. However, the increase in stability comes at the cost of the increase in the power required for the control system. [ABSTRACT FROM AUTHOR]

In this study, we investigate the feasibility of using a multiple-input multiple-output (MIMO) modal testing technique on an inflated torus. In particular, the refined modal testing methodology focuses on using Macro-Fiber Composite (MFC®) patches as both actuators and sensors. MFC® patches can be integrated in an unobtrusive way into the skin of the torus, and can be used to find a gossamer structure''s modal parameters. Furthermore, MFC® excitation produces less interference with the suspension modes of the free–free torus than excitations from a conventional shaker. The use of multiple actuators is shown to properly excite the global modes of the structure and distinguish between pairs of modes at nearly identical resonant frequencies. Formulation of the MIMO test as well as the required post-processing techniques are studied and successfully applied to an inflated Kapton® torus. [Copyright &y& Elsevier]

This paper presents the use of macro-fiber composites (MFC) for vibration suppression and structural health monitoring. The major advantages of the piezoelectric fiber composite actuators are their high performance, flexibility, and durability when compared with the traditional piezoceramic (PZT) actuators. The recently developed MFC actuator provides these advantages and can be used in structural vibration applications. In addition, the ability of MFC devices to couple the electrical and mechanical fields is larger than in monolithic PZT.In this study, we showed that an MFC could be used as a sensor and actuator to find modal parameters of an inflatable structure. This sensor and actuator combination has also been used to reduce vibration in an inflated object. Once the sensing capability was identified, we developed a self-sensing circuit for an MFC. Our experimental results clearly indicate that this strategy can suppress structural vibration, while reducing the number of system components. Finally, the MFC has been implemented as impedance sensor for structural health monitoring (e.g. a of bolted joint connection). The experimental results presented in this paper show the potential of MFC for use in the dynamics and control of flexible structures. [Copyright &y& Elsevier]

A novel method of guided wave-based structural health monitoring is developed in which no direct baseline data are required to identify structural damage. Conventional wave propagation structural health monitoring techniques involve the comparison of structural response data to a prerecorded baseline or reference measurement taken while the structure is in pristine condition. The need to compare new data to a prerecorded baseline can present several complications, including data management issues and difficulty in accommodating the effects of varying environmental and operational conditions on the data. To address the complications associated with baseline comparison, this new method accomplishes reference-free damage detection by acquiring what is referred to as an instantaneous baseline measurement for analysis. The instantaneous baseline technique is validated through both analytical and experimental testing. Analytical tests show that the instantaneous baseline method is able to correctly identify simulated damage. It is found experimentally that nonpermanent damage in the form of removable putty as well as permanent damage in the form of corrosion and cuts are all identifiable in thin aluminum plate test structures without direct comparison to baseline data when implementing the instantaneous baseline method. [ABSTRACT FROM AUTHOR]

Abstract: A major challenge impeding the deployment of wireless sensor networks for structural health monitoring (SHM) is developing a means to supply power to the sensor nodes in an efficient manner. In this paper, we explore possible solutions to this challenge by using a mobile-host based wireless energy transmission system to provide both power and data interrogation commands to sensor nodes. The mobile host features the capability of wirelessly transmitting energy to sensor nodes on an as-needed basis. In addition, it serves as a central data repository and processing center for the data collected from the sensing network. The wirelessly transmitted microwave energy is captured by a receiving antenna, transformed into DC power by a rectifying circuit, and stored in a storage medium to provide the required energy to the sensor node. The application of wireless energy transmission is targeted toward SHM sensor nodes that have been recently developed by the authors, which can be used to collect peak mechanical displacements or piezoelectric impedance measurements. This paper will describe considerations needed to design such energy transmission systems, experimental procedure and results, method of increasing the efficiency, energy conditioning circuits and storage medium, and target applications. Experimental results from a field test on the Alamosa Canyon Bridge in southern New Mexico will also be presented. [Copyright &y& Elsevier]

Abstract: A field demonstration of a new and hybrid wireless sensing network paradigm for structural health monitoring (SHM) is presented. In this paradigm, both power and data interrogation commands are conveyed via a mobile agent that is sent to each sensor node to perform individual interrogations, which can alleviate several limitations of traditional sensing networks. This paper will discuss such prototype systems, which will be used to interrogate capacitive-based and impedance-based sensors for SHM applications. The capacitive-based wireless sensor node is specifically built to collect peak displacement measurements. In addition, a wireless sensor node for collecting electromechanical impedance data has also been developed. Both sensor nodes are specifically designed to accept various power sources and to be wirelessly triggered on an as-needed basis so that they can be used for the hybrid sensing network approach. The capabilities of these miniaturized and portable devices are demonstrated in the laboratory and the field, which was performed at the Alamosa Canyon Bridge in southern New Mexico. [Copyright &y& Elsevier]