Showing papers by "Charles R. Farrar published in 2009"

Structural health monitoring algorithm comparisons using standard data sets

Abstract: The real-world structures are subjected to operational and environmental condition changes that impose difficulties in detecting and identifying structural damage. The aim of this report is to detect damage with the presence of such operational and environmental condition changes through the application of the Los Alamos National Laboratory’s statistical pattern recognition paradigm for structural health monitoring (SHM). The test structure is a laboratory three-story building, and the damage is simulated through nonlinear effects introduced by a bumper mechanism that simulates a repetitive impact-type nonlinearity. The report reviews and illustrates various statistical principles that have had wide application in many engineering fields. The intent is to provide the reader with an introduction to feature extraction and statistical modelling for feature classification in the context of SHM. In this process, the strengths and limitations of some actual statistical techniques used to detect damage in the structures are discussed. In the hierarchical structure of damage detection, this report is only concerned with the first step of the damage detection strategy, which is the evaluation of the existence of damage in the structure. The data from this study and a detailed description of the test structure are available for download at: http://institute.lanl.gov/ei/software-and-data/.

A Mobile Host Approach for Wireless Powering and Interrogation of Structural Health Monitoring Sensor Networks

Abstract: Wireless sensor networks (WSNs) for structural health monitoring (SHM) applications can provide the data collection necessary for rapid structural assessment after an event such as a natural disaster puts the reliability of civil infrastructure in question. Technical challenges affecting deployment of such a network include ensuring power is maintained at the sensor nodes, reducing installation and maintenance costs, and automating the collection and analysis of data provided by a wireless sensor network. In this work, a new "mobile host" WSN paradigm is presented. This architecture utilizes nodes that are deployed without resident power. The associated sensors operate on a mechanical memory principle. A mobile host, such as a robot or unmanned aerial vehicle, is used on an as-needed basis to charge the node by wireless power delivery and subsequently retrieve the data by wireless interrogation. The mobile host may be guided in turn to any deployed node that requires interrogation. The contribution of this work is the first field demonstration of a mobile host wireless sensor network. The sensor node, referred to as THINNER, capable of collecting data wirelessly in the absence of electrical power was developed. A peak displacement sensor capable of interfacing with the THINNER sensor node was also designed and tested. A wireless energy delivery package capable of being carried by an airborne mobile host was developed. Finally, the system engineering required to implement the overall sensor network was carried out. The field demonstration took place on an out-of-service, full-scale bridge near Truth-or-Consequences, NM.

Structural Health Monitoring With Autoregressive Support Vector Machines

Abstract: The use of statistical methods for anomaly detection has become of interest to researchers in many subject areas. Structural health monitoring in particular has benefited from the versatility of statistical damage-detection techniques. We propose modeling structural vibration sensor output data using nonlinear time-series models. We demonstrate the improved performance of these models over currently used linear models. Whereas existing methods typically use a single sensor's output for damage detection, we create a combined sensor analysis to maximize the efficiency of damage detection. From this combined analysis we may also identify the individual sensors that are most influenced by structural damage.

RF Energy Transmission for a Low-Power Wireless Impedance Sensor Node

Abstract: The proper management of energy resources is essential for any wireless sensing system. With applications that span industrial, civil, and aerospace infrastructure, it is necessary for sensors and sensor nodes to be physically robust and power efficient. In many applications, a sensor network must operate in locations that are difficult to access, and often these systems have a desired operational lifespan which exceeds that of conventional battery technologies. In the present study, the use of microwave energy is examined as an alternate method for powering compact, deployable wireless sensor nodes. A prototype microstrip patch antenna has been designed to operate in the 2.4 GHz ISM band and is used to collect directed radio frequency (RF) energy to power a wireless impedance device that provides active sensing capabilities for structural health monitoring applications. The system has been demonstrated in the laboratory, and was deployed in field experiments on the Alamosa Canyon Bridge in New Mexico in August 2007. The transmitted power was limited to 1 W in field tests, and was able to charge the sensor node to 3.6 V in 27 s. This power level was sufficient to measure two piezoelectric sensors and transmit data back to a base station on the bridge.

An Energy Harvesting Comparison of Piezoelectric and Ionically Conductive Polymers

Abstract: With advances in wireless communications and low power electronics there is an ever increasing need for efficient self-contained power systems. Traditional batteries are often selected for this purpose; however, there are limitations due to finite life-spans and the need to periodically recharge or replace the spent power source. One method to address this issue is the inclusion of an energy harvesting strategy that can scavenge energy from the surrounding environment and convert it into usable electrical energy. Since civil, industrial, and aerospace applications are often plagued with an overabundance of ambient vibrations, electromechanical transducers are often considered a viable choice for energy scavengers. In this study, two classes of transducer are considered: the piezoelectric polymer polyvinylidene fluoride and the ionically conductive ionic polymer transducer. Analytical models are formed for each material assuming axial loading and simulation results are compared with experimental results fo...

Piezoelectric Active-Sensor Diagnostics and Validation Using Instantaneous Baseline Data

Abstract: This paper presents a signal processing tool that efficiently performs piezoelectric (PZT) sensor diagnostic and validation. Validation of the sensor/actuator functionality during structural health monitoring (SHM) operation is a critical component to successfully implement a complete and robust SHM system, especially with an array of PZT active-sensors involved. The basis of this method is to track the capacitive value of PZT transducers, which manifests in the imaginary part of the measured electrical admittance. Both degradation of the mechanical/electrical properties of a PZT transducer and the bonding defects between a PZT patch and a host structure can be identified by the proposed process. However, it is found that the temperature variations in sensor boundary conditions manifest themselves in similar ways in the measured electrical admittances. Therefore, we examine the effects of temperature variation on the sensor diagnostic process and develop an efficient signal processing tool that enables the identification of a sensor validation feature that can be obtained instantaneously without relying on prestored baselines. This paper concludes with experimental results to demonstrate the effectiveness of the proposed technique.

Sensor Self-diagnosis Using a Modified Impedance Model for Active Sensing-based Structural Health Monitoring:

Abstract: The active sensing methods using piezoelectric materials have been extensively investigated for the efficient use in structural health monitoring (SHM) applications. Relying on high frequency structural excitations, the methods showed the extreme sensitivity to minor defects in a structure. Recently, a sensor self-diagnostic procedure that performs in situ monitoring of the operational status of piezoelectric (PZT) active sensors and actuators in SHM applications has been proposed. In this investigation, previously developed impedance models were revisited in order to investigate the effects of sensor and/or bonding defects on the admittance measurement. New parameters for sensor quality assessment of a PZT and coupling degradation effects between a PZT and bonding layer were incorporated into the traditional electromechanical impedance model for better estimation of the electromechanical impedance signatures and sensor diagnostics. The feasibility of the modified impedance model for sensor self-diagnosis...

A mobile-agent-based wireless sensing network for structural monitoring applications

Abstract: A new wireless sensing network paradigm is presented for structural monitoring applications. In this approach, both power and data interrogation commands are conveyed via a mobile agent that is sent to sensor nodes to perform intended interrogations, which can alleviate several limitations of the traditional sensing networks. Furthermore, the mobile agent provides computational power to make near real-time assessments on the structural conditions. This paper will discuss such prototype systems, which are used to interrogate impedance-based sensors for structural health monitoring applications. Our wireless sensor node is specifically designed to accept various energy sources, including wireless energy transmission, and to be wirelessly triggered on an as-needed basis by the mobile agent or other sensor nodes. The capabilities of this proposed sensing network paradigm are demonstrated in the laboratory and the field.

A low-power wireless sensing device for remote inspection of bolted joints:

Abstract: A new bolted-joint monitoring system is presented. This system consists of structural joint members equipped with piezoelectric (PZT) sensing elements and a wireless impedance device for data acquisition and communication. PZT enhanced washers are used to continuously monitor the condition of the joint by monitoring its dynamic characteristics. The mechanical impedance matching between the PZT enhanced devices and the joint connections is used as a key feature to monitor the preload changes and to prevent further failure. The dynamic response is readily measured using the electromechanical coupling property of the PZT patch, in which its electrical impedance is directly coupled with the mechanical impedance of the structure. A new miniaturized and portable impedance measuring device is implemented for the practical implementation of the proposed method. The proposed system can be used for the remote and rapid inspection of bolt tension and connection damage. Both theoretical modelling and experime...

Wireless impedance device for electromechanical impedance sensing and low-frequency vibration data acquisition

Abstract: This paper presents recent developments in an extremely compact, wireless impedance sensor node for combined use with both impedance method and low-frequency vibrational data acquisition. The sensor node, referred to as the WID3 (Wireless Impedance Device) integrates several components, including an impedance chip, a microcontroller for local computing, telemetry for wireless data transmission, multiplexers for managing up to seven piezoelectric transducers per node, energy storage mediums, and several triggering options into one package to truly realize a self-contained wireless active-sensor node for SHM applications. Furthermore, we recently extended the capability of this device by implementing low-frequency A/D and D/A converters so that the same device can measure low-frequency vibration data. The WID3 requires less than 60 mW of power to operate and is designed for the mobile-agent based wireless sensing network. The performance of this miniaturized device is compared to our previous results and its capabilities are demonstrated.

Powering Wireless SHM Sensor Nodes through Energy Harvesting

Abstract: The concept of wireless sensor nodes and sensor networks has been widely investigated for various applications, including the field of structural health monitoring (SHM). However, the ability to power sensors, on board processing, and telemetry components is a significant challenge in many applications. Several energy harvesting techniques have been proposed and studied to solve such problems. This chapter summarizes recent advances and research issues in energy harvesting relevant to the embedded wireless sensing networks, in particular SHM applications. A brief introduction of SHM is first presented and the concept of energy harvesting for embedded sensing systems is addressed with respect to various sensing modalities used for SHM and their respective power requirements. The power optimization strategies for embedded sensing networks are then summarized, followed by several example studies of energy harvesting as it has been applied to SHM embedded sensing systems. The paper concludes by defining some future research directions that are aimed at transitioning the concept of energy harvesting for embedded sensing systems from laboratory research to field-deployed engineering prototypes.

Piezoelectric Impedance Methods for Damage Detection and Sensor Validation

Abstract: This article presents an overview and recent applications in impedance-based structural health monitoring (SHM). The basic principle behind this technique is to apply high-frequency structural excitations (typically greater than 30 kHz) through surface-bonded piezoelectric transducers, and measure the local mechanical impedance by monitoring the current and voltage applied to the piezoelectric transducers. Changes in impedance indicate changes in the structure, which in turn can indicate that damage has occurred. Previous experimental studies are summarized to demonstrate how this technique can be efficiently used to detect structural damage in real time. The impedance method also has applications in the field of sensor self-diagnostics and validation in determining the operational status of piezoelectric active sensors used in SHM. The feasibility of this sensor validation procedure is demonstrated by analytical studies and experimental examples, where the functionality of surface-mounted piezoelectric sensors was continuously deteriorated. Keywords: damage detection; piezoelectric transducers; sensor validation; impedance method

Principles of Structural Degradation Monitoring

Abstract: This article describes a coherent strategy for the intelligent monitoring of structures and systems. All the relevant features of the strategy are discussed in detail. These encompass (i) a taxonomy for the relevant concepts, i.e., a precise definition of what, for example, constitutes a fault. (ii) a specification for operational evaluation that makes use of a hierarchical damage identification scheme, (iii) an approach to sensor prescription and optimization, and (iv) a data-processing methodology based on a data fusion model. A number of recently proposed “axioms” are summarized in the hope that they will shed light on the design of monitoring systems; a number of possible barriers to progress are also identified and discussed. Keywords: intelligent fault detection; holistic design; pattern recognition; axioms for health monitoring; barriers to implementation

Energy Harvesting and Wireless Energy Transmission for SHM Sensor Nodes

Abstract: The concept of wireless sensor nodes and sensor networks has been widely investigated within the field of structural health monitoring (SHM). However, the ability to power sensors, onboard processing, and telemetry components is a significant challenge in many SHM applications. Several energy harvesting techniques, based on piezoelectric and thermoelectric materials, have been proposed and studied to solve such problems. This article summarizes recent advances and research issues in energy harvesting relevant to SHM applications. We also present a feasibility study for using radio frequency (RF) signals to wirelessly deliver energy to power sensing systems. The goal of this study is to provide a method for supplying power to a sensor node periodically so that the SHM system does not rely on an onboard power source. Keywords: structural health monitoring (SHM); energy harvesting; wireless energy transmission; sensor networks; sensor nodes

Structural health monitoring for ship structures

Abstract: Currently the Office of Naval Research is supporting the development of structural health monitoring (SHM) technology for U.S. Navy ship structures. This application is particularly challenging because of the physical size of these structures, the widely varying and often extreme operational and environmental conditions associated with these ships missions, lack of data from known damage conditions, limited sensing that was not designed specifically for SHM, and the management of the vast amounts of data that can be collected during a mission. This paper will first define a statistical pattern recognition paradigm for SHM by describing the four steps of (1) Operational Evaluation, (2) Data Acquisition, (3) Feature Extraction, and (4) Statistical Classification of Features as they apply to ship structures. Note that inherent in the last three steps of this process are additional tasks of data cleansing, compression, normalization and fusion. The presentation will discuss ship structure SHM challenges in the context of applying various SHM approaches to sea trials data measured on an aluminum multi-hull high-speed ship, the HSV-2 Swift. To conclude, the paper will discuss several outstanding issues that need to be addressed before SHM can make the transition from a research topic to actual field applications onmore » ship structures and suggest approaches for addressing these issues.« less

An efficient signal processing tool for impedance-based structural health monitoring

Abstract: Various experimental studies have demonstrated that an impedance-based approach to structural health monitoring can be an effective means of damage detection. Using the self-sensing and active-sensing capabilities of piezoelectric materials, the electromechanical impedance response can be monitored to provide a qualitative indication of the overall health of a structure. Although impedance analyzers are commonly used to collect such data, they are bulky and impractical for long-term field implementation, so a smaller and more portable device is desired. However, impedance measurements often contain a sizeable number of data points, and a smaller device may not possess enough memory to store the required information, particularly for real-time analysis. Therefore, the amount of data used to assess the integrity of a structure must be significantly reduced. A new type of cross correlation analysis, for which impedance data is instantaneously correlated between different sensor sets and different frequency ranges, as opposed to be correlated to pre-stored baseline data, is proposed to drastically reduce the amount of data to a single correlation coefficient and provide a quantitative means of detecting damage relative to the sensor positions. The proposed analysis is carried out on a 3-story representative structure and its efficiency is demonstrated.

Energy harvesting and wireless energy transmission for embedded sensor nodes

Abstract: In this paper, we present experimental investigations using energy harvesting and wireless energy transmission to operate embedded 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 RF 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 discussed.

Sensor Network Paradigms

Abstract: Structural health monitoring (SHM) requires a rigorous approach to designing the sensing system used for the data-acquisition portion of a problem. To date, most SHM sensor system designs are done in a somewhat ad hoc manner where the engineer picks a sensing system that is readily available and that he or she is familiar with, and then attempts to demonstrate that a specific type of damage can be detected with that system. More recently, researchers have begun to develop sensor networks designed specially for SHM. On the basis of this research, several SHM sensor network paradigms have emerged, and this article is intended to provide an overview of these paradigms. First, a brief summary of the statistical pattern-recognition approach to SHM problem is presented. The data-acquisition portion of the paradigm is then addressed in detail, where the various parameters of the system that must be considered in its design and subsequent field deployment are summarized. This article focuses on several general SHM sensor network paradigms that have emerged and concludes by summarizing axioms for SHM that have been proposed in the literature and that are directly related to sensing issues for SHM. Keywords: structural health monitoring; sensor networks; sensor nodes

Multi-tiered sensing and data processing for monitoring ship structures

Abstract: A comprehensive structural health monitoring (SHM) system is a critical mechanism to ensure hull integrity and evaluate structural performance over the life of a ship, especially for lightweight high-speed ships. One of the most important functions of a SHM system is to provide real-time performance guidance and reduce the risk of structural damage during operations at sea. This is done by continuous feedback from onboard sensors providing measurements of seaway loads and structural responses. Applications of SHM should also include diagnostic capabilities such as identifying the presence of damage, assessing the location and extent of damage when it does occur in order to plan for future inspection and maintenance. The development of such SHM systems is extremely challenging because of the physical size of these structures, the widely varying and often extreme operational and environmental conditions associated with the missions of high performance ships, the lack of data from known damage conditions, the limited sensing that was not designed specifically for SHM, the management of the vast amounts of data, and the need for continued, real-time data processing. This paper will discuss some of these challenges and several outstanding issues that need to be addressed in the context of applyingmore » various SHM approaches to sea trials data measured on an aluminum high-speed catamaran, the HSV-2 Swift. A multi-tiered approach for sensing and data processing will be discussed as potential SHM architecture for future shipboard application. This approach will involve application of low cost and dense sensor arrays such as wireless communications in selected areas of the ship hull in addition to conventional sensors measuring global structural response of the ship. A recent wireless hull monitoring demo on FSF-I SeaFighter will be discussed as an example to show how this proposed architecture is a viable approach for long-term and real-time hull monitoring.« less

Developing an integrated software solution for active-sensing SHM

Abstract: A novel approach for integrating active sensing data interrogation algorithms for structural health monitoring (SHM) applications is presented. These algorithms cover Lamb wave propagation, impedance methods, and sensor diagnostics. Contrary to most active-sensing SHM techniques, which utilize only a single signal processing method for damage identification, a suite of signal processing algorithms are employed and grouped into one package to improve the damage detection capability. A MATLAB-based user interface, referred to as HOPS, was created, which allows the analyst to configure the data acquisition system and display the results from each damage identification algorithm for side-by-side comparison. By grouping a suite of algorithms into one package, this study contributes to and enhances the visibility and interpretation of the active-sensing methods related to damage identification. This paper will discuss the detailed descriptions of the damage identification techniques employed in this software and outline future issues to realize the full potential of this software.

Active-Sensing Lamb Wave Propagations for Damage Identification in Honeycomb Aluminum Panels

Abstract: This paper presents a novel approach for Lamb wave based structural health monitoring(SHM) in honeycomb aluminum panels. In this study, a suite of three signal processing algorithms are employed to improve the damage detection capability. The signal processing algorithms used include wavelet attenuation, correlation coefficients of power density spectra, and triangulation of reflected waves. Piezoelectric transducers are utilized as both sensors and actuators for Lamb wave propagation. These SHM algorithms are built into a MatLab interface that integrates and automates the hardware and software operations and displays the results for each algorithm to the analyst for side by side comparison. The effectiveness of each of these signal processing algorithms for SHM in honeycomb aluminum panels under a variety of damage conditions is then demonstrated.

Multiscale wireless sensor node for impedance-based SHM and low-frequency vibration data acquisition

Abstract: This paper presents recent developments in an extremely compact, wireless impedance sensor node (WID3, Wireless Impedance Device) at Los Alamos National Laboratory for use in impedance-based structural health monitoring (SHM), Sensor diagnostics and low-frequency vibrational data acquisition. The current generation WID3 is equipped with an Analog Devices AD5933 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 an Atmega1281 microcontroller, data storage using flash 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 and analog converters so that the same device can measure structural vibration data. The WID3 requires less than 70 mW of power to operate, and it can operate in various wireless network paradigms. The performance of this miniaturized and portable device is compared to our previous results and its broader capabilities are demonstrated.