A Review of Machine Vision-Based Structural Health Monitoring: Methodologies and Applications
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paper_title: Detection and classification of surface defects of gun barrels using computer vision and machine learning

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Abstract This work proposes a machine vision based approach for the detection and classification of the surface defects such as normal wear, corrosive pitting, rust and erosion that are usually present in used gun barrels. Surface images containing the defective regions of several used gun barrels were captured in a non-destructive manner using a Charge-Coupled Device (CCD) camera attached with a miniature microscopic probe. Among the captured images, normal wear appeared as bright and the rest of the three defects appeared as dark. Therefore, the classification has been carried out in two stages. Various segmentation methods were tested and extended maxima transform gave the best result. The defective area was calculated in metric units. Multiple textural features based on histogram and gray level co-occurrence matrix were extracted from the segmented images and ranked them automatically using the sequential forward feature selection method in order to select the best minimal features for the classification purpose. Many classifiers based on Bayes, k -Nearest Neighbor, Artificial Neural Network and Support Vector Machine (SVM) were tested and the results demonstrated the efficacy of SVM for this application. All these steps were carried out at six different scales of image sizes and the best scale was selected for the entire analysis based on the segmentation and classification accuracy. The introduction of this Gaussian scale spacing concept could reduce the computation without compromising on the accuracy. Overall, the methodology forms a novel framework for surface defect detection and classification that has a potential to automate the inspection process.
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paper_title: Image recognition technology in rotating machinery fault diagnosis based on artificial immune

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By using image recognition technology, this paper presents a new fault diagnosis method for rotating machinery with artificial immune algorithm. This method focuses on the vibration state parameter image. The main contribution of this paper is as follows: firstly, 3-D spectrum is created with raw vibrating signals. Secondly, feature information in the state parameter image of rotating machinery is extracted by using Wavelet Packet transformation. Finally, artificial immune algorithm is adopted to diagnose rotating machinery fault. On the modeling of 600MW turbine experimental bench, rotor`s normal rate, fault of unbalance, misalignment and bearing pedestal looseness are being examined. It`s demonstrated from the diagnosis example of rotating machinery that the proposed method can improve the accuracy rate and diagnosis system robust quality effectively.
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paper_title: Achievements and Challenges in Machine Vision-Based Inspection of Large Concrete Structures

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Large concrete structures need to be inspected in order to assess their current physical and functional state, to predict future conditions, to support investment planning and decision making, and to allocate limited maintenance and rehabilitation resources. Current procedures in condition and safety assessment of large concrete structures are performed manually leading to subjective and unreliable results, costly and time-consuming data collection, and safety issues. To address these limitations, automated machine vision-based inspection procedures have increasingly been proposed by the research community. This paper presents current achievements and open challenges in vision-based inspection of large concrete structures. First, the general concept of Building Information Modeling is introduced. Then, vision-based 3D reconstruction and as-built spatial modeling of concrete civil infrastructure are presented. Following that, the focus is set on structural member recognition as well as on concrete damage detection and assessment exemplified for concrete columns. Although some challenges are still under investigation, it can be concluded that vision-based inspection methods have significantly improved over the last 10 years, and now, as-built spatial modeling as well as damage detection and assessment of large concrete structures have the potential to be fully automated.
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paper_title: A Vision-Based Dynamic Rotational Angle Measurement System for Large Civil Structures

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In this paper, we propose a vision-based rotational angle measurement system for large-scale civil structures. Despite the fact that during the last decade several rotation angle measurement systems were introduced, they however often required complex and expensive equipment. Therefore, alternative effective solutions with high resolution are in great demand. The proposed system consists of commercial PCs, commercial camcorders, low-cost frame grabbers, and a wireless LAN router. The calculation of rotation angle is obtained by using image processing techniques with pre-measured calibration parameters. Several laboratory tests were conducted to verify the performance of the proposed system. Compared with the commercial rotation angle measurement, the results of the system showed very good agreement with an error of less than 1.0% in all test cases. Furthermore, several tests were conducted on the five-story modal testing tower with a hybrid mass damper to experimentally verify the feasibility of the proposed system.
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paper_title: Monitoring global earthquake-induced demands using vision-based sensors

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A vision-based approach is evaluated for its applicability as a new sensing technology for measuring earthquake-induced motions. The approach evaluated in this paper is advantageous since it requires very limited physical attachment to the structure of interest, is high-speed, high-resolution, and does not introduce additional mass or otherwise modify the properties of the structure. A demonstration experiment is described in which four digital high-speed, high-resolution, charge-coupled-device cameras outfitted with red light-emitting diodes are used to track 21 reflective (nearly) mass less spherical elements discretely mounted on a scale five-story steel frame structure. The structure is mounted on a large bi-axial shake table and subjected to different earthquake motions. A total of eleven conventional (wired) transducers [linear variable displacement transducers and accelerometers] are also discretely mounted on the structure, providing a unique comparison with the vision-based approach. Results from these experiments show that the nonintrusive vision-based approach is extremely promising in terms of its ability to capture inter-story drift, floor level velocities, and accelerations, provided proper post-processing of the dynamic data occurs.
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paper_title: An advanced vision-based system for real-time displacement measurement of high-rise buildings

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This paper introduces an advanced vision-based system for dynamic real-time displacement measurement of high-rise buildings using a partitioning approach. The partitioning method is based on the successive estimation of relative displacements and rotational angles at several floors using a multiple vision-based displacement measurement system. In this study, two significant improvements were made to realize the partitioning method: (1) time synchronization, (2) real-time dynamic measurement. Displacement data and time synchronization information are wirelessly transferred via a network using the TCP/IP protocol. The time synchronization process is periodically conducted by the master system to guarantee the system time at the master and slave systems are synchronized. The slave system is capable of dynamic real-time measurement and it is possible to economically expand measurement points at slave levels using commercial devices. To verify the accuracy and feasibility of the synchronized multi-point vision-based system and partitioning approach, many laboratory tests were carried out on a three-story steel frame model. Furthermore, several tests were conducted on a five-story steel frame tower equipped with a hybrid mass damper to experimentally confirm the effectiveness of the proposed system.
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paper_title: Horizontal Roadway Curvature Computation Algorithm Using Vision Technology

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Horizontal roadway curvature data is essential for roadway safety analysis. However, collecting such data is time-consuming, costly, and dangerous using traditional, manual surveying methods. It is especially difficult to perform such manual measurement when roadways have high traffic volumes. Thus, it would be valuable for transportation agencies if roadway curvature data could be computed from photographic images taken using low-cost digital cameras. This is the first article that develops an algorithm using emerging vision technology to acquire horizontal roadway curvature data from roadway images to perform roadway safety assessment. The proposed algorithm consists of 4 steps: 1) curve edges image processing, 2) mapping edge positions from an image domain to the real-world domain, 3) calibrating camera parameters, and 4) calculating the curve radius and center from curve points. The proposed algorithm was tested on roadways having various levels of curves and using different image sources to demonstrate its capability. The ground truth curvatures for 2 cases were also collected to evaluate the error of the proposed algorithm. The test results are very promising, and the computed curvatures are especially accurate for curves of small radii (less than 66 m/200 ft) with less than 1.0% relative errors with respect to the ground truth data. The proposed algorithm can be used as an alternative method that complements the traditional measurement methods used by state DOTs to collect roadway curvature data.
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paper_title: A preliminary study on the response of steel structures using surveillance camera image with vision-based method during the Great East Japan Earthquake

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Abstract Steel structures had experienced an intense and long transient response caused by the great earthquake motion during the Great East Japan Earthquake on 11th March 2011. Lots of non-structural damages such as ceiling fallen and glass broken are the main damage characteristics of steel structures although there were rarely structural damages. Some opened earthquake records and videos were adopted in this study in order to investigate the seismic behavior of the steel structures. An application of vision-based method has been carried out on a video of surveillance camera provided by Japan Broadcasting Corporation (Nippon Housou Kyoukai) to obtain the displacement response and natural period of the high-rise steel buildings in Tokyo.
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paper_title: Safety Monitoring of Railway Tunnel Construction Using FBG Sensing Technology

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In comparison with above-ground structures, the investigation of underground space structures still faces great challenges because of the extremely complicated constitutive relationships of the soils or rocks. Implementation of structural health monitoring (SHM) systems on the underground structures such as tunnels commencing from the construction stage may be of help in understanding their operational behaviors and long-term trends. This paper explores the application of the fiber Bragg grating (FBG) sensing technology for safety monitoring during railway tunnel construction. An FBG-based temperature monitoring system is first developed for real-time temperature measurement of the frozen soils during freezing construction of a metro-tunnel cross-passage. Through in-situ deployment of FBG-based liquid-level sensors, the subgrade settlement of a segment of a high-speed rail line is then monitored in an automatic manner during construction of an undercrossing tunnel. The field results indicate that the FBG ...
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paper_title: Statistical analysis of stress spectra for fatigue life assessment of steel bridges with structural health monitoring data

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Abstract This paper aims at developing a monitoring-based method for fatigue life assessment of steel bridges with use of long-term monitoring data of dynamic strain. A standard daily stress spectrum is derived by statistically analyzing the stress spectra accounting for highway traffic, railway traffic, and typhoon effects. The optimal number of daily strain data for derivation of the standard daily stress spectrum is determined by examining the predominant factors which affect the prediction of fatigue life. With the continuously measured dynamic strain responses from the instrumented Tsing Ma Bridge carrying both highway and railway traffic, the proposed method is exemplified to evaluate the fatigue life of fatigue-critical welded details on the bridge.
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paper_title: Multistep Explicit Stereo Camera Calibration Approach to Improve Euclidean Accuracy of Large-Scale 3D Reconstruction

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AbstractThe spatial accuracy of point clouds generated from a mobile stereo camera set is very sensitive to the intrinsic and extrinsic camera parameters (i.e., camera calibration) used in stereo image-based three-dimensional (3D) reconstruction methods. The existing camera calibration algorithms induce a significant amount of error owing to poor estimation accuracy in camera parameters when they are used for large-scale scenes such as mapping civil infrastructure. This leads to higher uncertainties in the location of 3D points, and may result in the failure of the whole reconstruction process. This paper proposes a novel procedure to address this problem. It hypothesizes that a set of multiple calibration parameters created by videotaping a moving calibration pattern along a specific path can increase overall calibration accuracy and ultimately enhance the Euclidean accuracy of the generated point cloud. This is achieved by using conventional camera calibration algorithms to perform separate estimations ...
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paper_title: A review on computer vision based defect detection and condition assessment of concrete and asphalt civil infrastructure

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Visual inspection of civil infrastructure is essential for condition assessment.We focus on concrete bridges, tunnels, underground pipes, and asphalt pavements.Accordingly, we review the latest computer vision based defect detection methods.Using computer vision most relevant types of defects can be automatically detected.Automatic defect properties retrieval and assessment has not been achieved yet. To ensure the safety and the serviceability of civil infrastructure it is essential to visually inspect and assess its physical and functional condition. This review paper presents the current state of practice of assessing the visual condition of vertical and horizontal civil infrastructure; in particular of reinforced concrete bridges, precast concrete tunnels, underground concrete pipes, and asphalt pavements. Since the rate of creation and deployment of computer vision methods for civil engineering applications has been exponentially increasing, the main part of the paper presents a comprehensive synthesis of the state of the art in computer vision based defect detection and condition assessment related to concrete and asphalt civil infrastructure. Finally, the current achievements and limitations of existing methods as well as open research challenges are outlined to assist both the civil engineering and the computer science research community in setting an agenda for future research.
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paper_title: Local positioning systems versus structural monitoring: a review

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SUMMARY ::: ::: Structural monitoring and structural health monitoring could take advantage from different devices to record the static or dynamic response of a structure. A positioning system provides displacement information on the location of moving objects, which is assumed to be the basic support to calibrate any structural mechanics model. The global positioning system could provide satisfactory accuracy in absolute displacement measurements. But the requirements of an open area position for the antennas and a roofed room for its data storage and power supply limit its flexibility and its applications. Several efforts are done to extend its field of application. The alternative is local positioning system. Non-contact sensors can be easily installed on existing infrastructure in different locations without changing their properties: several technological approaches have been exploited: laser-based, radar-based, vision-based, etc. In this paper, a number of existing options, together with their performances, are reviewed. Copyright © 2014 John Wiley & Sons, Ltd.
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paper_title: Multi-image stitching and scene reconstruction for evaluating defect evolution in structures

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It is well-recognized that civil infrastructure monitoring approaches that rely on visual approaches will continue to be an important methodology for condition assessment of such systems. Current inspection standards for structures such as bridges require an inspector to travel to a target structure site and visually assess the structure's condition. A less time-consuming and inexpensive alternative to current visual monitoring methods is to use a system that could inspect structures remotely and also more frequently. This article presents and evaluates the underlying technical elements for the development of an integrated inspection software tool that is based on the use of inexpensive digital cameras. For this purpose, digital cameras are appropriately mounted on a structure (e.g., a bridge) and can zoom or rotate in three directions (similar to traffic cameras). They are remotely controlled by an inspector, which allows the visual assessment of the structure's condition by looking at images captured by...
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paper_title: Yarn features extraction using image processing and computer vision: a study with cotton and polyester yarns

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Abstract The aim of this paper is the development of a new technological solution, for the automatic characterization of the yarn mass parameters (linear mass, diameter, and hairiness) based on image processing and computer vision techniques. A preliminary study for the detection and distinction between loop and protruding fibers is also presented. A custom-made application developed in LabVIEW from National Instruments with the IMAQ Vision Toolkit was used to acquire, analyze and process the yarn images. Some experimental results using cotton and polyester yarns were performed and compared with a commercial solution for validation. The presented approach allows a correct yarn parameterization improving products’ quality in the textile industry.
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paper_title: Experimental Study of Dynamic Crack Growth in Unidirectional Graphite/Epoxy Composites using Digital Image Correlation Method and High-speed Photography

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In this work, fracture behavior of multilayered unidirectional graphite/epoxy composite (T800/3900-2) materials is investigated. Rectangular coupons with a single-edged notch are studied under geometrically symmetric loading configurations and impact loading conditions. The notch orientation parallel to or at an angle to the fiber orientation is considered to produce mode-I or mixed-mode (mode-I and -II) fracture. Feasibility of studying stress-wave induced crack initiation and rapid crack growth in fiber-reinforced composites using the digital image correlation method and high-speed photography is demonstrated. Analysis of photographed random speckles on specimen surface provides information pertaining to crack growth history as well as surface deformations in the crack-tip vicinity. Measured deformation fields are used to estimate mixed-mode fracture parameters and examine the effect of fiber orientation (β) on crack initiation and growth behaviors. The samples show differences in fracture responses dep...
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paper_title: Close-range photogrammetry applications in bridge measurement: Literature review

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Abstract Close-range photogrammetry has found many diverse applications in the fields of industry, biomechanics, chemistry, biology, archaeology, architecture, automotive, and aerospace, as well as accident reconstruction. Although close-range photogrammetry has not been as popular in bridge engineering as in other fields, the investigations that have been conducted demonstrate the potential of this technique. The availability of inexpensive, off-the-shelf digital cameras and soft-copy, photogrammetry software systems has made close-range photogrammetry much more feasible and affordable for bridge engineering applications. To increase awareness of the use of this powerful non-contact, non-destructive technique in the bridge engineering field, this paper presents a literature review on the basic development of close-range photogrammetry and briefly describes previous work related to bridge deformation and geometry measurement; structural test monitoring; and historic documentation. The major aspects of photogrammetry bridge measurement are covered starting from the late 1970s and include a description of measurement types, cameras, targets, network control, and software. It is shown that early applications featured the use of metric cameras (specially designed for photogrammetry purposes), diffuse targets (non-retroreflective), stereoscopic photogrammetry network layout, and analog analytical tools, which transformed over time to the use of non-metric cameras, retro-reflective targets, highly convergent network layout, and digital computerized analytical tools.
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paper_title: Monitoring-Based Fatigue Reliability Assessment of Steel Bridges: Analytical Model and Application

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A fatigue reliability model which integrates the probability distribution of hot spot stress range with a continuous probabilistic formulation of Miner's damage cumulative rule is developed for fatigue life and reliability evaluation of steel bridges with long-term monitoring data. By considering both the nominal stress obtained by measurements and the corresponding stress concentration factor (SCF) as random variables, a probabilistic model of the hot spot stress is formulated with the use of the S-N curve and the Miner's rule, which is then used to evaluate the fatigue life and failure probability with the aid of structural reliability theory. The proposed method is illustrated using the long-term strain monitoring data from the instrumented Tsing Ma Bridge. A standard daily stress spectrum accounting for highway traffic, railway traffic, and typhoon effects is derived by use of the monitoring data. Then global and local finite element models (FEMs) of the bridge are developed for numerically calculating the SCFs at fatigue-susceptible locations, while the stochastic characteristics of SCF for a typical welded T-joint are obtained by full-scale model experiments of a railway beam section of the bridge. A multimodal probability density function (PDF) of the stress range is derived from the monitoring data using the finite mixed Weibull distributions in conjunction with a hybrid parameter estimation algorithm. The failure probability and reliability index versus fatigue life are achieved from the obtained joint PDF of the hot spot stress in terms of the nominal stress and SCF random variables.
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paper_title: Modeling of Stress Spectrum Using Long-Term Monitoring Data and Finite Mixture Distributions

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This study focuses on how to exploit long-term monitoring data of structural strain for analytical modeling of multimodal rainflow-counted stress spectra by use of the method of finite mixture distributions in conjunction with a hybrid mixture parameter estimation algorithm. The long-term strain data acquired from an instrumented bridge carrying both highway and railway traffic is used to verify the procedure. A wavelet-based filtering technique is first applied to eliminate the temperature effect inherent in the measured strain data. The stress spectrum is obtained by extracting the stress range and mean stress from the stress time histories with the aid of a rainflow counting algorithm. By synthesizing the features captured from daily stress spectra, a representative sample of stress spectrum accounting for multiple loading effects is derived. Then, the modeling of the multimodal stress range is performed by use of finite mixed normal, lognormal, and Weibull distributions, with the best mixed distributi...
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paper_title: Vision Metrology And Three Dimensional Visualization In Structural Testing And Monitoring

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This paper describes initial work carried out to advance the Erse of vision metrology and three dimensional visualization techniques in structural testing and monitoring. A description is given of a methodology for semi-automatically generating dynamic three dimensional computer graphics models, which is employed to provide a three dimensional spatial framework for conventional engineering instrumentation. The use of techniques developed for two structural measurement applications is explained and opportunities for current and future research are reported.
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paper_title: Dynamic displacement measurement of large-scale structures based on the Lucas–Kanade template tracking algorithm

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Abstract The development of optics and computer technologies enables the application of the vision-based technique that uses digital cameras to the displacement measurement of large-scale structures. Compared with traditional contact measurements, vision-based technique allows for remote measurement, has a non-intrusive characteristic, and does not necessitate mass introduction. In this study, a high-speed camera system is developed to complete the displacement measurement in real time. The system consists of a high-speed camera and a notebook computer. The high-speed camera can capture images at a speed of hundreds of frames per second. To process the captured images in computer, the Lucas–Kanade template tracking algorithm in the field of computer vision is introduced. Additionally, a modified inverse compositional algorithm is proposed to reduce the computing time of the original algorithm and improve the efficiency further. The modified algorithm can rapidly accomplish one displacement extraction within 1 ms without having to install any pre-designed target panel onto the structures in advance. The accuracy and the efficiency of the system in the remote measurement of dynamic displacement are demonstrated in the experiments on motion platform and sound barrier on suspension viaduct. Experimental results show that the proposed algorithm can extract accurate displacement signal and accomplish the vibration measurement of large-scale structures.
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paper_title: Systematic errors in digital image correlation due to undermatched subset shape functions

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Digital image correlation techniques are commonly used to measure specimen displacements by finding correspondences between an image of the specimen in an undeformed or reference configuration and a second image under load. To establish correspondences between the two images, numerical techniques are used to locate an initially square image subset in a reference image within an image taken under load. During this process, shape functions of varying order can be applied to the initially square subset. Zero order shape functions permit the subset to translate rigidly, while first-order shape functions represent an affine transform of the subset that permits a combination of translation, rotation, shear and normal strains.
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paper_title: Accurate and robust ROI localization in a camshift tracking application

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Camshift has been well accepted as one of the most popular methods for object tracking. However, it fails to address complex situations, such as similar color interference, object occlusion, and illumination changes. In this paper, we enhance Camshift to enable it to handle the above-mentioned problems. A two-dimensional (2D) histogram of the hue and luminance is used for the color features of the target. To reduce the influence from irrelevant background pixels, a Flood-fill operation is implemented. The obtained 2D target model can precisely describe the target as well as achromatic points. A similarity score is evaluated to prevent similar color interference. When a target's colors are not distinguishable from the background colors, motion information will contribute to the tracking task. Finally, an average rate change is adopted to maintain progressive but not abrupt changes in the window size. The proposed algorithm has been extensively tested. The results are satisfactory while maintaining the processing in real time.
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paper_title: A vision-based system for dynamic displacement measurement of long-span bridges: algorithm and verification

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Dynamic displacement of structures is an important index for in-service structural condition and behavior assessment, but accurate measurement of structural displacement for large-scale civil structures such as long-span bridges still remains as a challenging task. In this paper, a vision-based dynamic displacement measurement system with the use of digital image processing technology is developed, which is featured by its distinctive characteristics in non-contact, long-distance, and high-precision structural displacement measurement. The hardware of this system is mainly composed of a high-resolution industrial CCD (charge-coupled-device) digital camera and an extended-range zoom lens. Through continuously tracing and identifying a target on the structure, the structural displacement is derived through cross-correlation analysis between the predefined pattern and the captured digital images with the aid of a pattern matching algorithm. To validate the developed system, MTS tests of sinusoidal motions under different vibration frequencies and amplitudes and shaking table tests with different excitations (the El-Centro earthquake wave and a sinusoidal motion) are carried out. Additionally, in-situ verification experiments are performed to measure the mid-span vertical displacement of the suspension Tsing Ma Bridge in the operational condition and the cable-stayed Stonecutters Bridge during loading tests. The obtained results show that the developed system exhibits an excellent capability in real-time measurement of structural displacement and can serve as a good complement to the traditional sensors.
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paper_title: Vision-based structural displacement measurement: System performance evaluation and influence factor analysis

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Abstract In the past decade, the emerging machine vision-based measurement technology has gained great concerns among civil engineers due to its overwhelming merits of non-contact, long-distance, and high-resolution. A critical issue regarding to the measurement performance and accuracy of the vision-based system is how to identify and eliminate the systematic and unsystematic error sources. In this paper, a vision-based structural displacement measurement system integrated with a digital image processing approach is developed. The performance of the developed vision-based system is evaluated by comparing the results simultaneously obtained by the vision-based system and those measured by the magnetostrictive displacement sensor (MDS). A series of experiments are conducted on a shaking table to examine the influence factors which will affect the accuracy and stability of the vision-based system. It is demonstrated that illumination and vapor have a critical effect on the measurement results of the vision-based system.
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paper_title: Multi-point displacement monitoring of bridges using a vision-based approach

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To overcome the drawbacks of the traditional contact-type sensor for structural displacement measurement, the vision-based technology with the aid of the digital image processing algorithm has received increasing concerns from the community of structural health monitoring (SHM). The advanced vision-based system has been widely used to measure the structural displacement of civil engineering structures due to its overwhelming merits of non-contact, long-distance, and high-resolution. However, seldom currently-available vision-based systems are capable of realizing the synchronous structural displacement measurement for multiple points on the investigated structure. In this paper, the method for vision-based multi-point structural displacement measurement is presented. A series of moving loading experiments on a scale arch bridge model are carried out to validate the accuracy and reliability of the vision-based system for multi-point structural displacement measurement. The structural displacements of five points on the bridge deck are measured by the vision-based system and compared with those obtained by the linear variable differential transformer (LVDT). The comparative study demonstrates that the vision-based system is deemed to be an effective and reliable means for multi-point structural displacement measurement.
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paper_title: Image-based structural dynamic displacement measurement using different multi-object tracking algorithms

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With the help of advanced image acquisition and processing technology, the vision-based measurement methods have been broadly applied to implement the structural monitoring and condition identification of civil engineering structures. Many noncontact approaches enabled by different digital image processing algorithms are developed to overcome the problems in conventional structural dynamic displacement measurement. This paper presents three kinds of image processing algorithms for structural dynamic displacement measurement, i.e., the grayscale pattern matching (GPM) algorithm, the color pattern matching (CPM) algorithm, and the mean shift tracking (MST) algorithm. A vision-based system programmed with the three image processing algorithms is developed for multi-point structural dynamic displacement measurement. The dynamic displacement time histories of multiple vision points are simultaneously measured by the vision-based system and the magnetostrictive displacement sensor (MDS) during the laboratory shaking table tests of a three-story steel frame model. The comparative analysis results indicate that the developed vision-based system exhibits excellent performance in structural dynamic displacement measurement by use of the three different image processing algorithms. The field application experiments are also carried out on an arch bridge for the measurement of displacement influence lines during the loading tests to validate the effectiveness of the vision-based system.
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paper_title: Calibration methodology of a vision system for measuring the displacements of long-deck suspension bridges

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SUMMARY ::: ::: Structural health monitoring is an emergent powerful diagnostic tool that can be used to identify and to prevent possible failures of the various components that comprise an infrastructure. In a suspension bridge, the measurement of the vertical and transversal displacements plays an important role for its safety evaluation. Considering the restrictions usually found on these structures, an enhanced solution comprises a non-contact vision-based measurement system with dynamic response, accuracy and amplitude range well suited to the physical phenomenon to measure. ::: ::: ::: ::: A methodology to perform the vision system calibration is described, which can be carried out in situ, while the deck is moving, requiring little effort and a minimum set of information. The key idea is to use the factorization method to get an initial estimation of the object shape and camera's parameters, and to incorporate the knowledge about the distances between the calibration targets in a non-linear optimization process to achieve a metric shape reconstruction and optimize the camera's parameters estimation. ::: ::: ::: ::: Results obtained by numerical simulation experiments are reported, showing the negligible influence of noisy calibration data in the calibration performance and also the robustness of the calibration process on the camera's parameters estimation. Tests carried out to assess the calibration and tracking of the bridge deck motion showed that, even in an environmental severely affected by noise, it is possible to measure the vertical and transversal displacements of the bridge deck with excellent levels of accuracy. Copyright © 2011 John Wiley & Sons, Ltd.
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paper_title: Dynamic displacement measurement of large-scale structures based on the Lucas–Kanade template tracking algorithm

paper_content:

Abstract The development of optics and computer technologies enables the application of the vision-based technique that uses digital cameras to the displacement measurement of large-scale structures. Compared with traditional contact measurements, vision-based technique allows for remote measurement, has a non-intrusive characteristic, and does not necessitate mass introduction. In this study, a high-speed camera system is developed to complete the displacement measurement in real time. The system consists of a high-speed camera and a notebook computer. The high-speed camera can capture images at a speed of hundreds of frames per second. To process the captured images in computer, the Lucas–Kanade template tracking algorithm in the field of computer vision is introduced. Additionally, a modified inverse compositional algorithm is proposed to reduce the computing time of the original algorithm and improve the efficiency further. The modified algorithm can rapidly accomplish one displacement extraction within 1 ms without having to install any pre-designed target panel onto the structures in advance. The accuracy and the efficiency of the system in the remote measurement of dynamic displacement are demonstrated in the experiments on motion platform and sound barrier on suspension viaduct. Experimental results show that the proposed algorithm can extract accurate displacement signal and accomplish the vibration measurement of large-scale structures.
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paper_title: Vision-Based Displacement Sensor for Monitoring Dynamic Response Using Robust Object Search Algorithm

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This study develops a vision-based displacement measurement system for remote monitoring of vibration of large-size structures such as bridges and buildings. The system consists of one or multiple video cameras and a notebook computer. With a telescopic lens, the camera placed at a stationary point away from a structure captures images of an object on the structure. The structural displacement is computed in real time through processing the captured images. A robust object search algorithm developed in this study enables accurate measurement of the displacement by tracking existing features on the structure without requiring a conventional target panel to be installed on the structure. The efficacy of the system in remote measurement of dynamic displacements was demonstrated through a shaking table test and a field experiment on a long-span bridge.
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paper_title: Vertical Displacement Measurements for Bridges Using Optical Fiber Sensors and CCD Cameras — A Preliminary Study

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Bridge managers all over the world are always looking for simple ways to measure bridge vertical displacements for structural health monitoring. However, traditional methods to obtain such data are either tedious or expensive. There is a need to develop a simple, inexpensive, and yet practical method to measure bridge vertical displacements. This paper proposes two methods using either optical fiber (FBG) sensors or a charge-coupled-device (CCD) camera, respectively, for vertical displacement measurements of bridges. The FBG sensor method is based on the measured horizontal strains together with the identified curvature functions obtained by a self-developed FBG Tilt sensor. CCD cameras use a large number of pixels to form an image. The CCD camera method utilizes image processing techniques for pixel identification and subsequent edge detection. A preliminary study to validate the proposed methods in laboratory was presented. The tests include applying the methods to determine the vertical displacements s...
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paper_title: Vibration Monitoring of Multiple Bridge Points by Means of a Unique Vision-Based Measuring System

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Bridge static and dynamic vibration monitoring is a key activity for both safety and maintenance purposes. The development of vision-based systems allows to use this type of devices for remote estimation of a bridge vibration, simplifying the measuring system installation. The uncertainty of this type of measurements is strongly related to the experimental conditions (mainly the pixel-to-millimeters conversion, the target texture, the camera characteristics and the image processing technique). In this paper two different types of cameras are used to monitor the response of a bridge to a train pass-by. The acquired images are analyzed using three different image processing techniques (Pattern Matching, Edge Detection and Digital Image Correlation) and the results are compared with a reference measurement, obtained by a laser interferometer providing single point measurements. Tests with different zoom levels are shown and the corresponding uncertainty values are estimated. As the zoom level decreases it is possible not only to measure the displacement of one point of the bridge, but also to grab images from a wide structure portion in order to recover displacements of a large number of points in the field of view. The extreme final solution would be having wide area measurements with no targets, to make measurements really easy, with clear advantages, but also with some drawbacks in terms of uncertainty to be fully comprehended.
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paper_title: Remote sensing of building structural displacements using a microwave interferometer with imaging capability

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Phase interference of microwave images has been experimented for remote submillimeter-accuracy detection of structural displacements of a real-scale building, subject to tensional stress. The images are obtained by a synthetic-aperture interferometric radar, making use of continuous-wave step-frequency waveform. Phase information of the synthesized microwave images is exploited for detecting displacements of the illuminated structure.
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paper_title: Seismic structural displacement measurement using a line-scan camera: camera-pattern calibration and experimental validation

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This article examines high-speed, line-scan cameras as a robust and high-speed displacement sensor for a range of seismic monitoring applications. They have the additional benefit of requiring no invasive mechanisms or added processing to provide a high-resolution output measure, and do not interfere architecturally. Following the method proposed by Lim et al. for measuring foundation pile movements, multiple displacements and motions of any structure can be determined in real time at rates over 1 kHz using only one high-speed, line-scan camera and a special pattern. This resolution is more than sufficient for structural monitoring and control problems. Moreover, a novel edge tracking algorithm is proposed that enables high-resolution measurement of large motions using relatively low-resolution, line-scan cameras. Further, as the accuracy of the measurement results depends directly on camera-pattern calibration and satisfying the assumptions made by Lim et al., an easy-to-implement calibration procedure is developed that ensures the accuracy of the measurement results. Finally, versatility of the total measurement procedure is examined through both harmonic and random vibration experiments with a suite of different input motions applied to a computer-controlled cart. Comparing the input and the measured motions confirms that vision-based structural displacement measurement utilising a high-speed, line-scan camera offers a robust, high-resolution and low-cost means of measuring structural vibration displacements.
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paper_title: 3D displacement measurement model for health monitoring of structures using a motion capture system

paper_content:

Abstract Unlike 1D or 2D displacement measurement sensors, a motion capture system (MCS) can determine the movement of markers in any direction precisely. In addition, an MCS can overcome the limitations of the sampling frequency in 3D measurements by terrestrial laser scanning (TLS) and global positioning system (GPS). This paper presents a method to measure three dimensional (3D) structural displacements using a motion capture system (MCS) with a high accuracy and sampling rate. The MCS measures 2D coordinates of a number of markers with multiple cameras; these measurements are then used to calculate the 3D coordinates of markers. Therefore, unlike previous 1D or 2D displacement measurement sensors, the MCS can determine precisely the movement of markers in any direction. In addition, since the MCS cameras can monitor several markers, measurement points are increased by the addition of more markers. The effectiveness of the proposed model was tested by comparing the displacements measured in a free vibration experiment of a 3-story structure with a height of 2.1 m using both the MCS and laser displacement sensors.
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paper_title: Non-contact measurement of the dynamic displacement of railway bridges using an advanced video-based system

paper_content:

Abstract This article describes the development of a non-contact dynamic displacement measurement system for railway bridges based on video technology. The system, consisting of a high speed video camera, an optical lens, lighting lamps and a precision target, can perform measurements with acquisition frame rates ranging from 64 fps to 500 fps, and be articulated with other measurement systems, which promotes its integration in structural health monitoring (SHM) systems. Preliminary tests of the system have shown that the measurements’ precision can be affected by: (i) movements of the camera stand and, therefore, rigid supports should be used and the camera should be protected from air flows; (ii) the distortion of the field of view, caused by the flow of heat waves generated by IR incandescent lighting and, therefore, the operating time of the lamps should be limited. The system was used to measure the displacement of a railway bridge’s deck, induced by the passage of trains at speeds between 120 km/h and 180 km/h, yielding a very good agreement between the results of displacement measurement obtained with the video system and with a LVDT. The achieved precision was below 0.1 mm for distances from the camera to the target up to 15 m, and in the order of 0.25 mm for a distance of 25 m. The application of an image processing technique at subpixel level resulted in real precisions generally inferior to the theoretical precisions.
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paper_title: Speckle pattern quality assessment for digital image correlation

paper_content:

Abstract To perform digital image correlation (DIC), each image is divided into groups of pixels known as subsets or interrogation cells. Larger interrogation cells allow greater strain precision but reduce the spatial resolution of the data field. As such the spatial resolution and measurement precision of DIC are limited by the resolution of the image. In the paper the relationship between the size and density of speckles within a pattern is presented, identifying that the physical properties of a pattern have a large influence on the measurement precision which can be obtained. These physical properties are often overlooked by pattern assessment criteria which focus on the global image information content. To address this, a robust morphological methodology using edge detection is devised to evaluate the physical properties of different speckle patterns with image resolutions from 23 to 705 pixels/mm. Trends predicted from the pattern property analysis are assessed against simulated deformations identifying how small changes to the application method can result in large changes in measurement precision. An example of the methodology is included to demonstrate that the pattern properties derived from the analysis can be used to indicate pattern quality and hence minimise DIC measurement errors. Experiments are described that were conducted to validate the findings of morphological assessment and the error analysis.
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paper_title: Systematic errors in digital image correlation caused by intensity interpolation

paper_content:

Recently, digital image correlation as a tool for surface defor- mation measurements has found widespread use and acceptance in the field of experimental mechanics. The method is known to reconstruct displacements with subpixel accuracy that depends on various factors such as image quality, noise, and the correlation algorithm chosen. How- ever, the systematic errors of the method have not been studied in detail. We address the systematic errors of the iterative spatial domain cross- correlation algorithm caused by gray-value interpolation. We investigate the position-dependent bias in a numerical study and show that it can lead to apparent strains of the order of 40% of the actual strain level. Furthermore, we present methods to reduce this bias to acceptable lev- els. © 2000 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(00)00911-9)
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paper_title: Recognition of camera-captured low-quality characters using motion blur information

paper_content:

Camera-based character recognition has gained attention with the growing use of camera-equipped portable devices. One of the most challenging problems in recognizing characters with hand-held cameras is that captured images undergo motion blur due to the vibration of the hand. Since it is difficult to remove the motion blur from small characters via image restoration, we propose a recognition method without de-blurring. The proposed method includes a generative learning method in the training step to simulate blurred images by controlling blur parameters. The method consists of two steps. The first step recognizes the blurred characters based on the subspace method, and the second one reclassifies structurally similar characters using blur parameters estimated from the camera motion. We have experimentally proved that the effective use of motion blur improves the recognition accuracy of camera-captured characters.
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paper_title: Study of systematic errors in strain fields obtained via DIC using heterogeneous deformation generated by plastic FEA

paper_content:

In this article, systematic errors that arise in the derivation of strain fields based on displacement fields obtained by digital image correlation (DIC) are analysed. Attention is paid to errors that arise from different implementations of the DIC technique. In particular, we investigate the influence of the shape function, the interpolation order and the subset size on the derived strains. In addition, we focus on errors that can be directly attributed to the derivation of the strain fields, e.g. the strain-window size and the strain-window interpolation order. The errors are estimated using numerically deformed images that were obtained by imposing finite element (FE) displacement fields on an undeformed image yielding plastic deformation of the specimen. This FE procedure simulates realistic experimental heterogeneous deformations at various load steps. It is shown that the errors on the strain fields can be substantially reduced if conscious choices in the abovementioned implementations are made.
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paper_title: The errors in digital image correlation due to overmatched shape functions

paper_content:

In subset-based digital image correlation (DIC), a proper shape function must be chosen to approximate the underlying displacement field of the target subsets to ensure an accurate subset matching. Shape functions with varying orders of Taylor's expansions (e.g. zero-, first- and second-order) have been proposed. However, since the actual deformation occurring in the deformed subsets cannot be known a priori in most practical measurements, problems of mismatch (undermatched or overmatched) inevitably arise, which lead to additional errors in the measurement displacements. Although systematic errors due to undermatched shape functions have been thoroughly studied, the displacement errors associated with overmatched shape functions are still not sufficiently clear to us. In this paper, the systematic and random errors caused by the use of overmatched shape functions are first examined using numerical translation tests with precisely controlled subpixel motions. The results reveal that overmatched shape functions will not introduce additional systematic error but can induce increased random error, while the latter is negligibly small when a larger subset is used. Thus, it can be made explicit that second-order shape functions, capable of depicting more complex local deformation, can be used for practical DIC applications as a default, because this effectively eliminates the possible systematic error associated with the use of first-order shape functions, while the possibly increased random errors are small enough if using a relatively larger subset. Two sets of images with inhomogeneous deformation are also used to verify the accuracy of DIC measurements using second-order shape functions.
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paper_title: Blind Deblurring and Denoising of Images Corrupted by Unidirectional Object Motion Blur and Sensor Noise

paper_content:

Low light photography suffers from blur and noise. In this paper, we propose a novel method to recover a dense estimate of spatially varying blur kernel as well as a denoised and deblurred image from a single noisy and object motion blurred image. A proposed method takes the advantage of the sparse representation of double discrete wavelet transform-a generative model of image blur that simplifies the wavelet analysis of a blurred image-and the Bayesian perspective of modeling the prior distribution of the latent sharp wavelet coefficient and the likelihood function that makes the noise handling explicit. We demonstrate the effectiveness of the proposed method on moderate noise and severely blurred images using simulated and real camera data.
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paper_title: Dynamics-based motion de-blurring for a PZT-driven, compliant camera orientation mechanism

paper_content:

This paper proposes a method for removing motion blur from images captured by a fast-moving robot eye. Existing image techniques focused on recovering blurry images due to camera shake with long exposure time. In addition, previous studies relied solely on properties of the images or used external sensors to estimate a blur kernel, or point spread function PSF. This paper focuses on estimating a latent image from the blur images taken by the robotic camera orientation system. A PZT-driven, compliant camera orientation system was employed to demonstrate the effectiveness of this approach. Discrete switching commands were given to the robotic system to create a rapid point-to-point motion while suppressing the vibration with a faster response. The blurry images were obtained when the robotic system created a rapid point-to-point motion, like human saccadic motion. This paper proposes a method for estimating the PSF in knowledge of system dynamics and input commands, resulting in a faster estimation. The proposed method was investigated under various motion conditions using the single-degree-of-freedom camera orientation system to verify the effectiveness and was compared with other approaches quantitatively and qualitatively. The experiment results show that overall the performance metric of the proposed method was 27.77% better than conventional methods. The computation time of the proposed method was 50 times faster than that of conventional methods.
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paper_title: Assessment of measuring errors in DIC using deformation fields generated by plastic FEA

paper_content:

In this article, systematic errors that arise from different implementations of digital image correlation (DIC) techniques are analyzed. In particular, we investigate the influence of the adopted correlation function, the interpolation order, the shape function and the subset size on the derived displacements. These errors are estimated using numerically deformed images that were obtained by imposing finite element (FE) displacement fields on an undeformed image yielding plastic deformation of the specimen. This FE procedure simulates realistic experimental heterogeneous deformations at various load steps. It is shown that DIC is able to reproduce these displacements up to a satisfactory level if conscious choices in the above-mentioned implementations are made.
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paper_title: Lens distortion correction for digital image correlation by measuring rigid body displacement

paper_content:

A method of lens distortion correction is proposed in order to improve the measurement accuracy of digital image correlation for two-dimensional displacement measurement. The amounts of lens distortion are evaluated from displacement distributions obtained in a rigid body in-plane translation or rotation test. After detecting the lens distortion, its coefficient is determined using the method of least squares. Then, the corrected displacement distributions are obtained. The effectiveness of the proposed method is demonstrated by applying the correction method to an in-plane translation test and tension tests. The experimental results show that the proposed distortion correction method eliminates the effect of lens distortion from measured displacements.
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paper_title: Systematic errors in digital image correlation due to undermatched subset shape functions

paper_content:

Digital image correlation techniques are commonly used to measure specimen displacements by finding correspondences between an image of the specimen in an undeformed or reference configuration and a second image under load. To establish correspondences between the two images, numerical techniques are used to locate an initially square image subset in a reference image within an image taken under load. During this process, shape functions of varying order can be applied to the initially square subset. Zero order shape functions permit the subset to translate rigidly, while first-order shape functions represent an affine transform of the subset that permits a combination of translation, rotation, shear and normal strains.
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paper_title: Use of rigid-body motion for the investigation and estimation of the measurement errors related to digital image correlation technique

paper_content:

The aim of this work is to investigate the sources of errors related to digital image correlation (DIC) technique applied to strain measurements. The knowledge of such information is important before the measured kinematic fields can be exploited. After recalling the principle of DIC, some sources of errors related to this technique are listed. Both numerical and experimental tests, based on rigid-body motion, are proposed. These tests are simple and easy-to-implement. They permit to quickly assess the errors related to lighting, the optical lens (distortion), the CCD sensor, the out-of-plane displacement, the speckle pattern, the grid pitch, the size of the subset and the correlation algorithm. The errors sources that cannot be uncoupled were estimated by amplifying their contribution to the global error. The obtained results permit to address a classification of the error related to the used equipment. The paper ends by some suggestions proposed in order to minimize the errors.
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paper_title: Experimental Analysis of the Errors due to Polynomial Interpolation in Digital Image Correlation

paper_content:

Digital image correlation attempts to estimate displacement fields by digitally correlating two images acquired before and after motion. To do so, pixel intensity has to be interpolated at non-integer locations. The ideal interpolator is the sinc, but as it requires infinite support, it is not normally used and is replaced by polynomials. Polynomial interpolation produces visually appealing results but introduces positional errors in the signal, thus causing the digital image correlation algorithms to converge to incorrect results. In this work, an experimental campaign is described, that aims to characterise the errors introduced by interpolation, focusing in particular on the systematic error and the standard deviation of displacements.
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paper_title: The systematic error in digital image correlation induced by self-heating of a digital camera

paper_content:

The systematic strain measurement error in digital image correlation (DIC) induced by self-heating of digital CCD and CMOS cameras was extensively studied, and an experimental and data analysis procedure has been proposed and two parameters have been suggested to examine and evaluate this. Six digital cameras of four different types were tested to define the strain errors, and it was found that each camera needed between 1 and 2 h to reach a stable heat balance, with a measured temperature increase of around 10 ?C. During the temperature increase, the virtual image expansion will cause a 70?230 ?? strain error in the DIC measurement, which is large enough to be noticed in most DIC experiments and hence should be eliminated.
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paper_title: Study of optimal subset size in digital image correlation of speckle pattern images

paper_content:

This paper investigates the effect of subset size, associated with image pattern quality and subset displacement functions, on the accuracy of deformation measurements by digital image correlation(DIC). A concept of subset entropy is introduced in this work to quantify the subset image pattern quality for DIC analysis and its effectiveness was demonstrated by experimental studies. By employing white-light images with almost uniform subset entropy and first-order displacement functions, the effect of subset size on DIC analysis was investigated for the deformation cases of translation, uniform deformation, and simulated quadratic deformation, respectively. The results show that the chosen subset size must be large enough for precise displacement measurements when subset displacement functions match underlying actual deformation. On the other hand, optimal subset size in DIC for nonhomgeneous deformation measurements appears as a result of a tradeoff between the influence of random errors and systematic errors.
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paper_title: Motion-blur evaluation: A comparison of approaches

paper_content:

— In this paper, the results obtained from two independent evaluations of motion-blur effects with respect to the agreement between the two different approaches used, imaging and non-imaging, are analyzed. The measurements have been carried out in different laboratories by different operators without the prior intention of a subsequent analysis as presented here. The resulting data is analyzed to quantify the repeatability of each instrument and, in a second step, the comparability of results from the two approaches is investigated. The imaging approach used in these experiments is based on a stationary high-speed camera with temporal oversampling and numerical image-data processing to obtain the intensity distribution on the retina of an observer under the condition of smooth pursuit eye tracking. Results from that approach are compared to results obtained from the evaluation of step responses acquired with optical transient recorders by frame-period convolution. Measurements are carried out with a first LCD monitor with test patterns of both contrast polarities, with three velocities of translation, and four levels of gray. A second object of measurement is used for investigation of the effect of operator intervention in the process of evaluation of the imaging approach, especially on the determination of the reference levels that are needed for evaluation of the normalized blurred edge (NBE). Possible sources of uncertainties are identified for all approaches and instruments. Based on the analysis of that data, the practicability of step-response-based evaluations of the “blurred edge width/time” compared to the results obtained using the high-speed imaging approach, as long as there is no motion-dependent image processing, are confirmed.
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paper_title: Restoration of TDI camera images with motion distortion and blur

paper_content:

Abstract Platform movement during exposure of imaging system severely degrades image quality. In the case of Time delay and integration (TDI) camera, abnormal movements cause not only image blur but also distortion, for image Point Spread Function (PSF) is space-variant. In this paper, we present a motion degradation model of TDI image, and provide a method to restore such degraded image. While a TDI camera is imaging, it outputs images row by row (or line by line) along the scanning axis, and our method processes in the same track. We firstly calculate the space-invariant PSF of each row using the movement information of the TDI camera. Then, we substitute pixels of the row and the ones of their neighbor rows together with the PSF into standard Richardson–Lucy algorithm. By deconvoluting we get the restored pixels of the row. The same operations are executed for all rows of the degraded TDI image. Finally, a restored image can be reconstructed from those restored rows. Both simulated and experimental results prove the effectiveness of our method.
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paper_title: Adaptive subset offset for systematic error reduction in incremental digital image correlation

paper_content:

Abstract Digital image correlation (DIC) relies on a high correlation between the intensities in the reference image and the target image. When decorrelation occurs due to large deformation or viewpoint change, incremental DIC is utilized to update the reference image and use the correspondences in this renewed image as the reference points in subsequent DIC computation. As each updated reference point is derived from previous correlation, its location is generally of sub-pixel accuracy. Conventional subset which is centered at the point results in subset points at non-integer positions. Therefore, the acquisition of the intensities of the subset demands interpolation which is proved to introduce additional systematic error. We hereby present adaptive subset offset to slightly translate the subset so that all the subset points fall on integer positions. By this means, interpolation in the updated reference image is totally avoided regardless of the non-integer locations of the reference points. The translation is determined according to the decimal of the reference point location, and the maximum are half a pixel in each direction. Such small translation has no negative effect on the compatibility of the widely used shape functions, correlation functions and the optimization algorithms. The results of the simulation and the real-world experiments show that adaptive subset offset produces lower measurement error than the conventional method in incremental DIC when applied in both 2D-DIC and 3D-DIC.
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paper_title: Recovering ball motion from a single motion-blurred image

paper_content:

Motion blur often affects the ball image in photographs and video frames in many sports such as tennis, table tennis, squash and golf. In this work, we operate on a single calibrated image depicting a moving ball over a known background, and show that motion-blurred ball images, usually unwelcome in computer vision, bear more information than a sharp image. We provide techniques for extracting such information ranging from low-level image processing to 3D reconstruction, and present a number of experiments and possible applications, such as ball localization with speed and direction measurement from a single image, and ball trajectory reconstruction from a single long-exposure photograph.
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paper_title: Long Deck Suspension Bridge Monitoring: The Vision System Calibration Problem

paper_content:

: Structural Health Monitoring is an emergent powerful diagnostic tool that can be used to identify and to prevent possible failures of the various components that comprise an infrastructure. In the particular case of a suspension bridge, the measurement of the vertical and transversal displacements plays an important role for its safety evaluation. Taking into account the restrictions usually found on these structures, an enhanced solution comprises a non-contact vision-based measurement system with dynamic response, accuracy and amplitude range well suited to the physical phenomenon to measure. The paper describes a methodology to perform the vision system calibration that can be carried out in-situ, while the deck is moving, requiring little effort and a minimum set of information. Specifically, only a set of active targets fixed on the deck and the knowledge about the distances between them is required. Results related to the performance evaluation, obtained by numerical simulation and by real experiments with a reduced structure model, are presented and they show that, even in an environment severely affected by disturbance noise, it is possible to measure the vertical and transversal displacements with a standard accuracy better than 10 mm.
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paper_title: Study of image characteristics on digital image correlation error assessment

paper_content:

In this paper, errors related to digital image correlation (DIC) technique applied to measurements of displacements are estimated. This work is based on the generation of synthetic images representative of real speckle patterns. With these images, various parameters are treated in order to determine their impact on the measurement error. These parameters are related to the type of deformation imposed on the speckle, the speckle itself (encoding of the image and image saturation) or the software (subset size).
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paper_title: Error estimation in measuring strain fields with DIC on planar sheet metal specimens with a non-perpendicular camera alignment

paper_content:

Abstract The determination of strain fields based on displacement components obtained via 2D-DIC is subject to several errors that originate from various sources. In this contribution, we study the impact of a non-perpendicular camera alignment to a planar sheet metal specimen's surface. The errors are estimated in a numerical experiment. To this purpose, deformed images – that were obtained by imposing finite element (FE) displacement fields on an undeformed image – are numerically rotated for various Euler angles. It is shown that a 3D-DIC stereo configuration induces a substantial compensation for the introduced image-plane displacement gradients. However, higher strain accuracy and precision are obtained – up to the level of a perfect perpendicular alignment – in a proposed “rectified” 2D-DIC setup. This compensating technique gains benefit from both 2D-DIC (single camera view, basic amount of correlation runs, no cross-camera matching nor triangulation) and 3D-DIC (oblique angle compensation). Our conclusions are validated in a real experiment on SS304.
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paper_title: Measurement of sinusoidal vibration from motion blurred images

paper_content:

Previous vision-based methods usually measure vibration from large sequence of unblurred images recorded by high-speed video or stroboscopic photography. In this paper, we propose a novel method for sinusoidal vibration measurement based on motion blurred images. We represent the motion blur information in images by the relationship between the geometric moments of the motion blurred images and the motion, and estimate the vibration parameters from this motion blur cue. We need only one motion blurred image and an unblurred image or two successive frames of blurred images to calculate the parameters of low-frequency vibration as well as the amplitude and direction of high-frequency vibration, while unblurred-image-based techniques rely on much more images to obtain the same results and existing motion-blurred-image-based approaches only estimate the amplitude of high-frequency vibration. Experimental results with both simulated and real vibrations of low and high frequencies are employed to demonstrate the effectiveness of the proposed scheme.
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paper_title: Quality assessment of speckle patterns for digital image correlation

paper_content:

Digital image correlation (DIC) is an optical-numerical full-field displacement measuring technique, which is nowadays widely used in the domain of experimental mechanics. The technique is based on a comparison between pictures taken during loading of an object. For an optimal use of the method, the object of interest has to be covered with painted speckles. In the present paper, a comparison is made between three different speckle patterns originated by the same reference speckle pattern. A method is presented for the determination of the speckle size distribution of the speckle patterns, using image morphology. The images of the speckle patterns are numerically deformed based on a finite element simulation. Subsequently, the displacements are measured with DIC-software and compared to the imposed ones. It is shown that the size of the speckles combined with the size of the used pixel subset clearly influences the accuracy of the measured displacements.
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paper_title: Dynamic testing of a laboratory model via vision-based sensing

paper_content:

In the class of not-contact sensors, the techniques of vision-based displacement estimation enable one to gather dense global measurements of static deformation as well as of dynamic response. They are becoming more and more available thanks to the ongoing technology developments. In this work, a vision system, which takes advantage of fast-developing digital image processing and computer vision technologies and provides high sample rate, is implemented to monitor the 2D plane vibrations of a reduced scale frame mounted on a shaking table as available in a laboratory. The physical meanings of the camera parameters, the trade-off between the system resolution and the field-of-view, and the upper limitation of marker density are discussed. The scale factor approach, which is widely used to convert the image coordinates measured by a vision system in the unit of pixels into space coordinates, causes a poor repeatability of the experiment, an unstable experiment precision, and therefore a global poor flexibility. To overcome these problems, two calibrations approaches are introduced: registration and direct linear transformation. Based on the constructed vision-based displacement measurement system, several experiments are carried out to monitor the motion of a scale-reduced model on which dense markers are glued. The experiment results show that the proposed system can capture and successfully measure the motion of the laboratory model within the required frequency band.
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paper_title: Use of Digital Image Processing in the Monitoring of Deformations in Building Structures.

paper_content:

Digital image processing was applied to develop a simple, robust and economical measuring system for detecting deformations in building structures. Selected points of the structure are captured by electronic cameras at regular intervals and their positions are measured in the digital image. The target points, in this case light-emitting diodes (LEDs), as well as the cameras used for measuring operate in the infrared range, which means that the quality of the images is not affected by the lighting conditions. Tests in the laboratory showed that using this technique it is possible to track targets over distances typical for building structures within a matter of millimeters. The practical applicability of the method was verified in a pilot project, for which the gym of the Staffelsee High School in Murnau in the district of Garmisch-Partenkirchen was chosen. The wide-span timber roof structure of the building was equipped with a system based on digital image processing gauging the movement of three points on each of the four main beams. Reference measurements can be taken with a built-in laser gauge. To be able to observe the correlation between weather conditions and deformation, the system is complemented by a weather station with snow-cushions on the roof of the building. Furthermore, the monitoring system is connected to multiple alarming devices.
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paper_title: Real-Time Displacement Measurement of a Flexible Bridge Using Digital Image Processing Techniques

paper_content:

In this study, real-time displacement measurement of bridges was carried out by means of digital image processing techniques. This is innovative, highly cost-effective and easy to implement, and yet maintains the advantages of dynamic measurement and high resolution. First, the measurement point is marked with a target panel of known geometry. A commercial digital video camera with a telescopic lens is installed on a fixed point away from the bridge (e.g., on the coast) or on a pier (abutment), which can be regarded as a fixed point. Then, the video camera takes a motion picture of the target. Meanwhile, the motion of the target is calculated using image processing techniques, which require a texture recognition algorithm, projection of the captured image, and calculation of the actual displacement using target geometry and the number of pixels moved. Field tests were carried out for the verification of the present method. The test results gave sufficient dynamic resolution in amplitude as well as the frequency. Use of this technology for a large suspension bridge is discussed considering the characteristics of such bridges having low natural frequencies within 3 Hz and the maximum displacement of several centimeters.
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paper_title: A simple image-based strain measurement method for measuring the strain fields in an RC-wall experiment

paper_content:

A simple image-based method for measuring plane strain fields on the surface of specimens in earthquake engineering experiments was developed. This method integrated camera calibration, stereo triangulation, image metric rectification and image template matching techniques to develop a method that was cost-effective, easy to apply and provided a satisfactory level of measurement accuracy. A zero-strain test conducted using this method showed that the measurement accuracy achieved was 0.04 pixels. That is, the relative displacement accuracy achieved was 0.005 mm and the strain accuracy was 0.001. This level of accuracy was achieved using eight-mega-pixel digital cameras to measure a 17 cm × 28 cm measurement region. Cracks that were 0.012 mm wide were identified in the concrete by examining the displacement fields calculated through the application of this image-based method in an RC-wall experiment. Copyright © 2011 John Wiley & Sons, Ltd.
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paper_title: A vision-based approach for the direct measurement of displacements in vibrating systems

paper_content:

This paper reports the results of an analytical and experimental study to develop, calibrate, implement and evaluate the feasibility of a novel vision-based approach for obtaining direct measurements of the absolute displacement time history at selectable locations of dispersed civil infrastructure systems such as long-span bridges. The measurements were obtained using a highly accurate camera in conjunction with a laser tracking reference. Calibration of the vision system was conducted in the lab to establish performance envelopes and data processing algorithms to extract the needed information from the captured vision scene. Subsequently, the monitoring apparatus was installed in the vicinity of the Vincent Thomas Bridge in the metropolitan Los Angeles region. This allowed the deployment of the instrumentation system under realistic conditions so as to determine field implementation issues that need to be addressed. It is shown that the proposed approach has the potential of leading to an economical and robust system for obtaining direct, simultaneous, measurements at several locations of the displacement time histories of realistic infrastructure systems undergoing complex three-dimensional deformations.
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paper_title: Monitoring of a civil structure’s state based on noncontact measurements

paper_content:

In this article, the comparison of two noncontact measurement methods dedicated to civil engineering structures’ state examination is presented. The vision-based method computes the displacement field of the analyzed structure by means of the digital image correlation coefficient. The system consists of one or more high-resolution digital cameras mounted on a head or on portable tripods. The developed methodology and created software application embedded in an MS-Windows operating system are presented. The second system measures the deflection of the structures by means of a radar interferometer. In both cases, it is possible to measure many points on the structure simultaneously. This article presents a comparison of the displacement field measurement performed on a field setup, as well as the span of a steel bridge designed for tram traffic. Both systems are described, with special attention given to their application in measurements of civil engineering structures. This article demonstrates a prelimina...
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paper_title: A Synchronized Multipoint Vision-Based System for Displacement Measurement of Civil Infrastructures

paper_content:

This study presents an advanced multipoint vision-based system for dynamic displacement measurement of civil infrastructures. The proposed system consists of commercial camcorders, frame grabbers, low-cost PCs, and a wireless LAN access point. The images of target panels attached to a structure are captured by camcorders and streamed into the PC via frame grabbers. Then the displacements of targets are calculated using image processing techniques with premeasured calibration parameters. This system can simultaneously support two camcorders at the subsystem level for dynamic real-time displacement measurement. The data of each subsystem including system time are wirelessly transferred from the subsystem PCs to master PC and vice versa. Furthermore, synchronization process is implemented to ensure the time synchronization between the master PC and subsystem PCs. Several shaking table tests were conducted to verify the effectiveness of the proposed system, and the results showed very good agreement with those from a conventional sensor with an error of less than 2%.
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paper_title: Analysis of the structural behavior of a membrane using digital image processing

paper_content:

Abstract This article presents a methodology for the experimental analysis of thin membranes using digital image processing techniques. The methodology is particularly suitable for structures that cannot be monitored using conventional systems, particularly those systems that involve contact with the structure. This methodology consists of a computer vision system that integrates the digital image acquisition and processing techniques on-line using special programming routines. Because the membrane analyzed is very thin and displays large displacements/strains, the use of any conventional sensor based on contact with the structure would not be possible. The methodology permits the measurement of large displacements at several points of the membrane simultaneously, thus enabling the acquisition of the global displacement field. The accuracy of the acquired displacement field is a function of the number of cameras and measured points. The second step is to estimate the strains and stresses on the membrane from the measured displacements. The basic idea of the methodology is to generate global two-dimensional functions that describe the strains and stresses at any point of the membrane. Two constitutive models are considered in the analysis: the Hookean and the neo-Hookean models. Favorable comparisons between the experimental and numerical results attest to the accuracy of the proposed experimental procedure, which can be used for both artificial and natural membranes.
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paper_title: A vision-based system for remote sensing of bridge displacement

paper_content:

This study proposed the vision-based system which remotely measures dynamic displacement of bridges in real-time using digital image processing techniques. This system has a number of innovative features including a high resolution in dynamic measurement, remote sensing, cost-effectiveness, real-time measurement and visualization, ease of installation and operation and no electro-magnetic interference. The digital video camera combined with a telescopic device takes a motion picture of the target installed on a measurement location. Meanwhile, the displacement of the target is calculated using an image processing technique, which requires a target recognition algorithm, projection of the captured image, and calculation of the actual displacement using target geometry and number of pixels moved. For the purpose of verification, a laboratory test using shaking table test and field application on a bridge with open-box girders were carried out. The test results gave sufficient dynamic resolution in frequency as well as the amplitude.
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paper_title: Structural dynamic displacement vision system using digital image processing

paper_content:

Abstract This study introduces dynamic displacement vision system (DDVS), which is applicable for imaging unapproachable structures using a hand-held digital video camcorder and is more economical than the existing contact and contactless measurement methods of dynamic displacement and deformation. This proposed DDVS method is applied to the Region of Interest (ROI) resizing and coefficient updating at each time step to improve the accuracy of the measurement from the digital image. Thus, after evaluating the algorithms conducted in this study by the static and dynamic verification, the measurement's usability by calculating the dynamic displacement of the masonry specimen, and the two-story steel frame specimen is evaluated under uniaxial seismic loading. The algorithm of the proposed method in this study, despite the relatively low resolution during frozen, slow, and seismic motions, has precision and usability that can replace the existing displacement transducer. Moreover, the method can be effectively applied to even fast behavior for multi-measurement positions like the seismic simulation test using large scale specimen. DDVS, using the consecutive images of the structures with an economic, hand-held digital video camcorder is a more economical displacement sensing concept than the existing contact and contactless measurement methods.
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paper_title: NONCONTACT PHOTOGRAMMETRIC MEASUREMENT OF VERTICAL BRIDGE DEFLECTION

paper_content:

This paper reports on the results from a study of vertical deflection measurement of bridges using digital close-range terrestrial photogrammetry (DCRTP). The study consisted of a laboratory and two field exercises. In the laboratory exercise, photogrammetric measurements of a 11.6 m (38 ft) steel beam loaded at midspan were made and compared with dial gauge readings and elastic beam theory. In the first field exercise, the initial camber and dead load deflection of 31.1 m (102 ft) prestressed concrete bridge girders were measured photogrammetrically and compared with level rod and total station readings. A comparison of the photogrammetric measurements with the dead load deflection diagram is also made. In the second field exercise, the vertical deflection of a 14.9 m (49 ft) noncomposite steel girder bridge loaded with two dump trucks was measured. Photogrammetric results are compared with deflections estimated using elastic finite-element analysis, level rod readings, and curvature-based deflection measurements. The paper is concluded with a discussion of work in progress to further improve the accuracy of DCRTP in the field.
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paper_title: A Vision-Based Sensor for Noncontact Structural Displacement Measurement

paper_content:

Conventional displacement sensors have limitations in practical applications. This paper develops a vision sensor system for remote measurement of structural displacements. An advanced template matching algorithm, referred to as the upsampled cross correlation, is adopted and further developed into a software package for real-time displacement extraction from video images. By simply adjusting the upsampling factor, better subpixel resolution can be easily achieved to improve the measurement accuracy. The performance of the vision sensor is first evaluated through a laboratory shaking table test of a frame structure, in which the displacements at all the floors are measured by using one camera to track either high-contrast artificial targets or low-contrast natural targets on the structural surface such as bolts and nuts. Satisfactory agreements are observed between the displacements measured by the single camera and those measured by high-performance laser displacement sensors. Then field tests are carried out on a railway bridge and a pedestrian bridge, through which the accuracy of the vision sensor in both time and frequency domains is further confirmed in realistic field environments. Significant advantages of the noncontact vision sensor include its low cost, ease of operation, and flexibility to extract structural displacement at any point from a single measurement.
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paper_title: BRIDGE DEFLECTION MEASUREMENT USING DIGITAL IMAGE CORRELATION

paper_content:

Digital image correlation technique is used for measuring vertical deflections of bridge girders during a bridge load testing. A bridge is loaded by a heavy cargo truck on the bridge road. Then, the deflection distribution is measured by digital image correlation. The applicability of digital image correlation to bridge deflection measurement is investigated by comparing the results obtained by digital image correlation with those obtained by displacement transducers. The effect of random pattern on an object surface is also investigated by measuring with and without random pattern. The results show that the deflection distributions of the bridge obtained by digital image correlation agree well with those obtained by the displacement transducers when the random pattern is attached on the bridge surface. In addition, it is found that the deflections can be measured even if the artificial random pattern is not applied to the surface of the bridge girder. It is emphasized that noncontact displacement measurement is possible by simple and easy procedure with digital image correlation for the structural evaluation of infrastructures.
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paper_title: Vision-based displacement measurement method for high-rise building structures using partitioning approach

paper_content:

The horizontal displacement of a high-rise building structure is usually considered as one of the major indicators to assess the structural safety. It is, however, difficult to directly measure the displacement of such a flexible structure due to the inaccessibility to a reference point usually needed for conventional displacement sensors and the huge size of the structure. In order to resolve this issue, a novel vision-based displacement measurement technique is proposed by employing a partitioning approach (i.e., successive estimation of relative displacements and rotational angles throughout a large flexible structure). A series of the experimental tests using two webcams installed on a flexible steel column structure have been conducted to validate the feasibility of the proposed method. The test results showed that the difference between the displacement measured from the proposed method and the exact value is less than 0.5%. Therefore, the proposed method could be considered as one promising candidate for measuring the displacement of high-rise building structures.
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paper_title: Vision-based algorithms for damage detection and localization in structural health monitoring

paper_content:

Summary ::: ::: Deflection curve can be used to detect and localize damage in civil engineering structures. In this paper, a vision-based method applied for in-plane displacement field measurement of cantilever beams is presented. The deflection curve of the analyzed structure is computed by means of the digital image correlation. Damage is introduced into the structure. Resulting deflection curves are used as an input to the novel damage detection algorithms: line segments method and voting method. The algorithms are then compared with the second derivative method and assessed through the probability of detection procedure. Copyright © 2015 John Wiley & Sons, Ltd.
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paper_title: Investigation of the dynamic characteristic of bridge structures using a computer vision method

paper_content:

A new method for the investigation of the dynamic characteristic of bridges has been developed. It is based on the photogrammetric principle; however, the viewing system is equipped with an additional reference system, which decreases the sensitivity to vibrations and an analysis system which enables image analysis. The method is used for monitoring and real-time measurement of the displacement of chosen points at bridge structures. It has applications particularly in the case of the measurement of hard-to-access places on bridges. The instrumentation, methodology and engineering examples of its application are presented.
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paper_title: An optical approach to structural displacement measurement and its application

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A number of optical devices are commercially available now for measurement. Some of them, such as laser devices and still and video cameras with high resolution, may be used effectively and efficiently for measuring structural displacement. This paper presents an approach to this type of application using a charge-coupled-device camera. It can acquire digitized images for low cost, to be used to identify structural displacement via digital signal processing. It is shown that this approach’s resolution for point measurement is comparable with traditional sensors such as dial gages. Furthermore, it offers a new capability of displacement measurement for a large number of points on a structure, and it can provide spatially intensive displacement data. This kind of data may be used for structural damage detection and health monitoring, as suggested and demonstrated herein.
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paper_title: Flexible Videogrammetric Technique for Three-Dimensional Structural Vibration Measurement

paper_content:

A videogrammetric technique is proposed for measuring three-dimensional structural vibration response in the laboratory. The technique is based on the principles of close-range digital photogrammetry and computer vision. Two commercial-grade digital video cameras are used for image acquisition. To calibrate these cameras and to overcome potential lens distortion problems, an innovative two-step calibration process including individual and stereo calibration is proposed. These calibrations are efficiently done using a planar pattern arbitrarily shown at a few different orientations. This special characteristic makes it possible to perform an on-site calibration that provides flexibility in terms of using different camera settings to suit various application conditions. To validate the proposed technique, three tests, including sinusoidal motion of a point, wind tunnel test of a cross-section bridge model, and a three-story building model under earthquake excitation, are performed. Results indicate that the proposed videogrammetric technique can provide fairly accurate displacement measurement for all three tests. The proposed technique is shown to be a good complement to the traditional sensors for measuring two- or three-dimensional vibration response in the low-frequency range.
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paper_title: Development and application of a vision-based displacement measurement system for structural health monitoring of civil structures

paper_content:

For structural health monitoring (SHM) of civil infrastructures, displacement is a good descriptor of the structural behavior under all the potential disturbances. However, it is not easy to measure displacement of civil infrastructures, since the conventional sensors need a reference point, and inaccessibility to the reference point is sometimes caused by the geographic conditions, such as a highway or river under a bridge, which makes installation of measuring devices time-consuming and costly, if not impossible. To resolve this issue, a visionbased real-time displacement measurement system using digital image processing techniques is developed. The effectiveness of the proposed system was verified by comparing the load carrying capacities of a steel-plate girder bridge obtained from the conventional sensor and the present system. Further, to simultaneously measure multiple points, a synchronized vision-based system is developed using master/slave system with wireless data communication. For the purpose of verification, the measured displacement by a synchronized vision-based system was compared with the data measured by conventional contact-type sensors, linear variable differential transformers (LVDT) from a laboratory test.
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paper_title: Monocular Computer Vision Method for the Experimental Study of Three-Dimensional Rocking Motion

paper_content:

AbstractThe rocking problem is applicable to a wide variety of structural and nonstructural elements. The current applications range from bridge pier and shallow footing design to hospital and data center equipment, even art preservation. Despite the increasing number of theoretical and simulation studies of rocking motion, few experimental studies exist. Of those that have been published, most are focused on a reduced version of the problem introducing modifications to the physical problem with the purpose of eliminating either sliding, uplift, or the three-dimensional (3D) response of the body. However, all of these phenomena may affect the response of an unrestrained rocking body. The intent of this work is to present a computer vision method that allows for the experimental measurement of the rigid body translation and rotation time histories in three dimensions. Experimental results obtained with this method will be presented to demonstrate that it obtains greater than 97% accuracy when compared agai...
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paper_title: A paired visual servoing system for 6-DOF displacement measurement of structures

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In the previous study, a paired structured light system which incorporates lasers, cameras, and screens was proposed, and experimental tests validated the potential of a displacement measurement system for large structures. However, the estimation of relative translational and rotational displacements between two sides was based on an assumption that there is zero initial displacement and that three laser beams are always on the screens. In this paper, a calibration method is proposed to offset the initial displacement using the first captured image. The calibration matrix derived from the initial offset is used for subsequent displacement estimation. A newly designed 2-DOF manipulator for each side is visually controlled to prevent the laser beams from leaving the screen. As the manipulator actively controls the laser beams to target the center of the screen, it contains all three laser points within the bounds of the screen. To verify the feasibility of the proposed system, various simulations and experimental tests were performed. The results show that the proposed visually servoed paired structured light system solves the main problem with the former system and that it can be utilized to enlarge the estimation range of the displacement.
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paper_title: A four-camera videogrammetric system for 3-D motion measurement of deformable object

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Abstract A four-camera videogrammetric system with large field-of-view is proposed for 3-D motion measurement of deformable object. Four high-speed commercial-grade cameras are used for image acquisition. Based on close-range photogrammetry, an accurate calibration method is proposed and verified for calibrating the four cameras simultaneously, where a cross target as calibration patterns with feature points pasted on its two-sides is used. The key issues of the videogrammetric processes including feature point recognition and matching, 3-D coordinate and displacement reconstruction, and motion parameters calculation are discussed in detail. Camera calibration experiment indicates that the proposed calibration method, with a re-projection error less than 0.05 pixels, has a considerable accuracy. Accuracy evaluation experiments prove that the accuracy of the proposed system is up to 0.5 mm on length dynamic measurement within 5000 mm×5000 mm field-of-view. Motion measurement experiment on an automobile tire is conducted to validate performance of our system. The experimental results show that the proposed four-camera videogrammetric system is available and reliable for position, trajectory, displacement and speed measurement of deformable moving object.
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paper_title: A new low-cost displacements monitoring system based on Kinect sensor

paper_content:

We present a new system for structural health monitoring. In particular, the reason we have developed the system is related to structural monitoring during (potentially) destructive tests. Structural response is usually sensed by sensors such as accelerometers, LVDTs and all the instruments which need to be placed in contact with the structure in order to work properly. This can cause problems during (potentially) destructive tests, like the ones performed on shaking tables, an important class of experiments aimed at monitoring the behavior of certain structures under the action of a seismic force. Thus, in the present paper, we focus our attention on designing a new displacements monitoring system, which does not need to be in direct contact with the structure in order to work. The goal is to use a really promising technology in the field of 3D space monitoring to build a system suitable to replace classical sensors such as LVDTs and accelerometers, usually adopted in displacements monitoring. This technology is based on computer vision and low-cost sensors; this allows us to protect monitoring systems from damage due to contact with the structure under testing. Results show that our system is able to estimate the position of the selected points on a test structure with good accuracy, with respect to a traditional high-precision sensor.
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paper_title: A New Position Measurement System Using a Motion-Capture Camera for Wind Tunnel Tests

paper_content:

Considering the characteristics of wind tunnel tests, a position measurement system that can minimize the effects on the flow of simulated wind must be established. In this study, a motion-capture camera was used to measure the displacement responses of structures in a wind tunnel test, and the applicability of the system was tested. A motion-capture system (MCS) could output 3D coordinates using two-dimensional image coordinates obtained from the camera. Furthermore, this remote sensing system had some flexibility regarding lab installation because of its ability to measure at relatively long distances from the target structures. In this study, we performed wind tunnel tests on a pylon specimen and compared the measured responses of the MCS with the displacements measured with a laser displacement sensor (LDS). The results of the comparison revealed that the time-history displacement measurements from the MCS slightly exceeded those of the LDS. In addition, we confirmed the measuring reliability of the MCS by identifying the dynamic properties (natural frequency, damping ratio, and mode shape) of the test specimen using system identification methods (frequency domain decomposition, FDD). By comparing the mode shape obtained using the aforementioned methods with that obtained using the LDS, we also confirmed that the MCS could construct a more accurate mode shape (bending-deflection mode shape) with the 3D measurements.
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paper_title: Using Specific Displacements to Analyze Motion without Calibration

paper_content:

Considering the field of un-calibrated image sequences and self-calibration, this paper analyzes the use of specific displacements (such as fixed axis rotation, pure translations,...) or specific sets of camera parameters. This allows to induce affine or metric constraints, which can lead to self-calibration and 3D reconstruction.A uniformed formalism for such models already developed in the literature plus some novel models are developed here. A hierarchy of special situations is described, in order to tailor the most appropriate camera model to either the actual robotic device supporting the camera, or to tailor the fact we only have a reduced set of data available.This visual motion perception module leads to the estimation of a minimal 3D parameterization of the retinal displacement for a monocular visual system without calibration, and leads to self-calibration and 3D dynamic analysis.The implementation of these equations is analyzed and experimented.
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paper_title: A stereoscopic digital speckle photography system for 3-D displacement field measurements

paper_content:

Stereoscopic digital speckle photography offers a technique to measure object shapes and 3-D displacement fields in experimental mechanics. The system measures the displacement of a random white light speckle pattern, which somehow is present on the object surface, using digital correlation. This paper describes a general physical model for stereo imaging systems. A camera calibration algorithm, which takes the distortion in the lenses into account, is also presented and evaluated by real experiments. Standard deviations of small deformations as low as 1% of the pixel size for in-plane deformations and 6% of the pixel size for the out-of-plane component are reported. Using the calibration algorithm described, the main source of errors is random errors originating from the correlation algorithm.
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paper_title: Three-Dimensional Structural Translation and Rotation Measurement Using Monocular Videogrammetry

paper_content:

Measuring displacement for large-scale structures has always been an important yet challenging task. In most applications, it is not feasible to provide a stationary platform at the location where its displacements need to be measured. Recently, image-based technique for three-dimensional (3D) displacement measurement has been developed and proven to be applicable to civil engineering structures. Most of these developments, however, use two or more cameras and require sophisticated calibration using a total station. In this paper, we present a single-camera approach that can simultaneously measure both 3D translation and rotation of a planar target attached on a structure. The intrinsic parameters of the camera are first obtained using a planar calibration board arbitrarily positioned around the target location. The obtained intrinsic parameters establish the relationship between the 3D camera coordinates and the two-dimensional image coordinates. These parameters can then be used to extract the rotation and translation of the planar target using recorded image sequence. The proposed technique is illustrated using two laboratory tests and one field test. Results show that the proposed monocular videogrammetric technique is a simple and effective alternative method to measure 3D translation and rotation for civil engineering structures. It should be noted that the proposed technique cannot measure translation along the direction perpendicular to the image plane. Hence, proper caution should be taken when placing target and camera.
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paper_title: Nontarget Stereo Vision Technique for Spatiotemporal Response Measurement of Line-Like Structures

paper_content:

With continuous advancement in optical, electronics, and computer technology, commercial digital cameras now are equipped with a high pixel resolution at a reasonable cost. Image sequences recorded by these cameras contain both spatial and temporal informa- tion of the target object; hence they can be used to extract the object's dynamic responses and characteristics. This paper presents a nontarget stereo vision technique to measure response of a line-like structure simultaneously in both spatial and temporal domain. The technique uses two digital cameras to acquire image sequences of a line-like structure. It adopts a simple nondimensional length matching approach and the epipolar geometry to establish point correspondences within an image sequence and between two image sequences, respectively. After reconstructing a spatio-temporal displacement response from the two image sequences, wavelet transform is then used to extract the modal characteristics of the structure. The technique is illustrated using two free vibration tests: a steel cantilever beam and a bridge stay cable. Results show that the technique can measure the spatiotemporal responses, natural frequencies, and mode shapes of the two structures quite accurately. This image-based technique is a low-cost and simple-to-use approach that not only can complement current response measurement sensors but also offer special advantages which are hard to obtain using these traditional sensors.
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paper_title: 3D displacement measurement model for health monitoring of structures using a motion capture system

paper_content:

Abstract Unlike 1D or 2D displacement measurement sensors, a motion capture system (MCS) can determine the movement of markers in any direction precisely. In addition, an MCS can overcome the limitations of the sampling frequency in 3D measurements by terrestrial laser scanning (TLS) and global positioning system (GPS). This paper presents a method to measure three dimensional (3D) structural displacements using a motion capture system (MCS) with a high accuracy and sampling rate. The MCS measures 2D coordinates of a number of markers with multiple cameras; these measurements are then used to calculate the 3D coordinates of markers. Therefore, unlike previous 1D or 2D displacement measurement sensors, the MCS can determine precisely the movement of markers in any direction. In addition, since the MCS cameras can monitor several markers, measurement points are increased by the addition of more markers. The effectiveness of the proposed model was tested by comparing the displacements measured in a free vibration experiment of a 3-story structure with a height of 2.1 m using both the MCS and laser displacement sensors.
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paper_title: Three-Dimensional Acceleration Measurement Using Videogrammetry Tracking Data

paper_content:

In order to evaluate the feasibility of multi-point, non-contact, acceleration measurement, a high-speed, precision videogrammetry system has been assembled from commercially-available components and software. Consisting of three synchronized 640 × 480 pixel monochrome progressive scan CCD cameras each operated at 200 frames per second, this system has the capability to provide surface-wide position-versus-time data that are filtered and twice-differentiated to yield the desired acceleration tracking at multiple points on a moving body. The oscillating motion of targets mounted on the shaft of a modal shaker were tracked, and the accelerations calculated using the videogrammetry data were compared directly to conventional accelerometer measurements taken concurrently. Although differentiation is an inherently noisy operation, the results indicate that simple mathematical filters based on the well-known Savitzky and Golay algorithms, implemented using spreadsheet software, remove a significant component of the noise, resulting in videogrammetry-based acceleration measurements that are comparable to those obtained using the accelerometers.
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paper_title: Experimental validation of non-contacting measurement method using LED-optical displacement sensors for vibration stress of small-bore piping

paper_content:

Abstract There have been many reports of fatigue failures of small-bore piping systems such as drain piping, vent piping and instrumentation piping in nuclear power plants that arise from vibration sources such as pumps. To prevent the failures, integrity evaluation of piping is conducted by measuring and analyzing vibration stress in the piping. But, a more efficient and economical measurement method is desirable to evaluate the vibration fatigue in small-bore piping. In this study, a non-contacting measurement method was proposed that is based on optical displacement sensors using light emission diodes (LEDs) to measure the vibration stress. The applicability of the method was discussed based on the vibration experiments using pipe elements and a mock-up piping system. From the experimental results, the proposed method was clarified to be sufficiently applicable and practically useful for the vibration measurement and stress evaluation in small-bore piping systems.
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paper_title: High resolution digital image correlation measurements of strain accumulation in fatigue crack growth

paper_content:

Abstract Microstructure plays a key role in fatigue crack initiation and growth. Consequently, measurements of strain at the microstructural level are crucial to understanding fatigue crack behavior. The few studies that provide such measurements have relatively limited resolution or areas of observation. This paper provides quantitative, full-field measurements of plastic strain near a growing fatigue crack in Hastelloy X, a nickel-based superalloy. Unprecedented spatial resolution for the area covered was obtained through a novel experimental technique based on digital image correlation (DIC). These high resolution strain measurements were linked to electron backscatter diffraction (EBSD) measurements of grain structure (both grain shape and orientation). Accumulated plastic strain fields associated with fatigue crack growth exhibited inhomogeneities at two length scales. At the macroscale, the plastic wake contained high strain regions in the form of asymmetric lobes associated with past crack tip plastic zones. At high magnification, high resolution DIC measurements revealed inhomogeneities at, and below, the grain scale. Effective strain not only varied from grain to grain, but also within individual grains. Furthermore, strain localizations were observed in slip bands within grains and on twin and grain boundaries. A better understanding of these multiscale heterogeneities could help explain variations in fatigue crack growth rate and crack path and could improve the understanding of fatigue crack closure and fracture in ductile metals.
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paper_title: Assessing steel strains on reinforced concrete members from surface cracking patterns

paper_content:

Abstract The measurement of steel strains in reinforced concrete structures is often critical to characterise the stresses along the member. In this scope, this manuscript describes the development of a method to assess steel strains inside concrete members using solely surface measurements. These measurements were obtained using photogrammetry and image processing. The technique was validated using two concrete ties monitored with strain gauges placed inside the steel bars. The experimental results showed that one of the most important parameters affecting the accuracy of the technique is the measurement of crack widths. In comparison, the concrete strain has little effect on the final results. The technique is particularly advantageous since it is non-contact and does not impact on the bond conditions. It also does not require accessing the reinforcements. As a main conclusion, this work showed the feasibility of estimating strains inside the structure using surface measurements. This technique will benefit in the near future from further improvements namely in what concerns the camera resolution.
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paper_title: New Parameters to Describe High-Temperature Deformation of Prestressing Steel Determined Using Digital Image Correlation

paper_content:

AbstractThis paper describes the results from a series of high-temperature tension tests on prestressing steel under sustained load (creep tests). Both steady-state and transient heating regimes ar...
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paper_title: Computer Vision-Based Technique to Measure Displacement in Selected Soil Tests

paper_content:

The paper investigates the accuracy of normal case photography as a means of measuring linear deformations of soil specimens. The results of using this approach were compared with results from conventional procedures for two soil tests, namely unconfined compression and direct shear tests. Charge-coupled-device (CCD) video cameras were used to measure deformation or strain in soil specimens. A personal computer based digital vision system was used to obtain accurately measured linear displacement data. Using remolded soil specimens, comparisons between displacement measurements using American Society for Testing and Materials (ASTM) conventional methods and normal case photography methods showed that use of the latter method is promising and could be used as a substitute for strain gages. Experimental investigation showed that differences between displacement measurements using conventional ASTM procedures and the computer vision technique were consistently within 0.04 plus or minus 0.15 to 0.3 plus or minus 0.23 mm for unconfined compression tests and direst shear tests, respectively. This was compatible with the image scale where one pixel on the image domain was equivalent to about 0.4 mm on object space coordinates. Statistical correlations between strains by the two methods supported this result. Image scale and resolution were found to be the two major factors affecting the accuracy of the measurements. The results of this work can further the search for more fully automated soil testing measurements.
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paper_title: Full-field measurements of heterogeneous deformation patterns on polymeric foams using digital image correlation

paper_content:

The ability of a digital image correlation technique to capture the heterogeneous deformation fields appearing during compression of ultra-light open-cell foams is presented in this article. Quantitative characterization of these fields is of importance to understand the mechanical properties of the collapse process and the energy dissipation patterns in this type of materials. The present algorithm is formulated in the context of multi-variable non-linear optimization where a merit function based on a local average of the deformation mapping is minimized implicitly. A parallel implementation utilizing message passing interface for distributed-memory architectures is also discussed. Estimates for optimal size of the correlation window based on measurement accuracy and spatial resolution are provided. This technique is employed to reveal the evolution of the deformation texture on the surface of open-cell polyurethane foam samples of different relative densities. Histograms of the evolution of surface deformation are extracted, showing the transition from unimodal to bimodal and back to unimodal. These results support the interpretation that the collapse of light open-cell foams occurs as a phase transition phenomenon.
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paper_title: Application of three-dimensional digital image correlation to the core-drilling method

paper_content:

We present a non-destructive technique for the determination ofin situ stresses in concrete structures, reterred to as the core-drilling method. The method is similar to the American Society for Testing and Materials (ASTM) hole-drilling strain gage method, except that the core-drilling method is formulated in the current work are performed with traditional photogrammetry, and the more novel (and more accurate) three-dimensional digital image correlation. In this paper we review the background elasticity theory and we discuss the results of verification experiments on steel plates. Calculated normal stresses are within 17% of applied values for photogrammetry, and 7% for three-dimensional digital image correlation.
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paper_title: Image-based stress and strain measurement of wood in the split-Hopkinson pressure bar

paper_content:

The properties of wood must be considered when designing mechanical pulping machinery. The composition of wood within the annual ring is important. This paper proposes a novel image-based method to measure stress and planar strain distribution in soft, heterogeneous materials. The main advantage of this method in comparison to traditional methods that are based on strain gauges is that it captures local strain gradients and not only average strains. Wood samples were subjected to compression at strain rates of 1000–2500 s−1 in an encapsulated split-Hopkinson device. High-speed photography captured images at 50 000–100 000 Hz and different magnifications to achieve spatial resolutions of 2.9 to 9.7 µm pixels−1. The image-based analysis utilized an image correlation technique with a method that was developed for particle image velocimetry. The image analysis gave local strain distribution and average stress as a function of time. Two stress approximations, using the material properties of the split-Hopkinson bars and the displacement of the transmitter bar/sample interface, are presented. Strain gauges on the bars of the split-Hopkinson device give the reference average stress and strain. The most accurate image-based stress approximation differed from the strain gauge result by 5%.
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paper_title: Calibration and evaluation of optical systems for full-field strain measurement

paper_content:

The design and testing of a reference material for the calibration of optical systems for strain measurement is described, together with the design and testing of a standardized test material that allows the evaluation and assessment of fitness for purpose of the most sophisticated optical system for strain measurement. A classification system for the steps in the measurement process is also proposed and allows the development of a unified approach to diagnostic testing of components or sub-systems in an optical system for strain measurement based on any optical technique. The results described arise from a European study known as SPOTS whose objectives were to begin to fill the gap caused by a lack of standards.
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paper_title: Measurement of Local Deformations in Steel Monostrands Using Digital Image Correlation

paper_content:

The local deformation mechanisms in steel monostrands have a significant influence on their fatigue life and failure mode. However, the observation and quantification of deformations in monostrands experiencing axial and transverse deformations is challenging because of their complex geometry, difficulties with the placement of strain gauges in the vicinity of the anchorage, and, most importantly, the relatively small magnitude of deformation occurring in the monostrand. This paper focuses on the measurement of localized deformations in high-strength steel monostrands using the digital image correlation (DIC) technique. The presented technique enables the measurement of individual wire strains along the length of the monostrand and also provides quantitative information on the relative movement between individual wires, leading to a more in-depth understanding of the underlying fatigue mechanisms. To validate the proposed image-based measurement method, two different tests were performed, with the one correlation method showing good agreement. Data collected from the DIC technique creates a basis for the analysis of the fretting and localized bending behavior of the monostrand and provides relevant information on the internal state of displacement of the monostrand under bending load.
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paper_title: On the use of digital image correlation for slip measurement during coupon scale fretting fatigue experiments

paper_content:

Abstract Digital image correlation (DIC) is a full field three dimensional measurement technique that can quantify displacements and strains of a surface. In this paper, digital image correlation is used as a slip measurement technique during coupon scale fretting fatigue experiments. Slip measured with the novel DIC technique is compared to conventional slip measurement techniques as clip gauges and modified clip gauge measurements proposed by Wittkowsky et al. Slip measurements with the DIC system show lower slip values and higher tangential contact stiffness’s compared to (modified) clip gauge measurements. Slip measured with DIC is obtained closer to the contact compared to clip gauges, eliminating the influence of elastic deformations or fitting parameters. During the fretting fatigue experiments are two equal contacts simultaneously tested. However, the slip of both contacts is not identical with outliers of more than 10% difference in slip amplitude.
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paper_title: Application of digital photogrammetry techniques in identifying the mode shape ratios of stay cables with multiple camcorders

paper_content:

Abstract A simple digital photogrammetry technique using artificial targets is developed to conduct the synchronized ambient vibration measurements at different locations of a stay cable with multiple camcorders for the identification of mode shape ratios. To compensate the restrictions imposed by the adoption of budget equipment, elaborate adjustments in scale, synchronization, and baselines are performed. Based on the on-site measurements for three stay cables, it is verified that the proposed methodology can effectively measure the ambient vibration responses of stay cables and attain the same order of accuracy in the identification of mode shape ratios as that of high-resolution velocimeters.
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paper_title: Cost‐effective vision‐based system for monitoring dynamic response of civil engineering structures

paper_content:

This study develops a cost-effective vision-based displacement measurement system for real-time monitoring of dynamic responses of large-size civil engineering structures such as bridges and buildings. The system simply consists of a low-cost digital camcorder and a notebook computer equipped with digital image-processing software developed in this study. A target panel with predesigned marks is attached to a structure, whose movement is captured by the digital camcorder placed at a fixed point away from the measurement point. The captured images are streamed to the notebook computer and processed by the software to compute displacement in real time. The efficacy of the system in measuring dynamic responses was demonstrated through laboratory tests, seismic shaking table tests on a steel building frame, and a field experiment on a bridge. In order to simultaneously measure displacements of multiple points, this study further developed a time synchronization system, in which TCP/IP protocol is employed for communications. The effectiveness of the time synchronization system was also experimentally verified. The vision-based system developed in this study is simple, cost-effective, easy to install, and capable of real-time measurement of dynamic responses, making the system ideal for monitoring civil engineering structures. Copyright © 2009 John Wiley & Sons, Ltd.
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paper_title: Nontarget Image-Based Technique for Small Cable Vibration Measurement

paper_content:

In this paper, a proof-of-concept image-based technique is proposed for measuring small cable vibration. The technique analyzes an image sequence of a vibrating cable segment captured by a camera. An optical flow method is used to calculate variation of optical intensity of an arbitrary selected region of interest ROI on the cable image sequence. The obtained optical flow vector provides the direction of vibration for the ROI on the cable segment, which then can be used to estimate displacement of the ROI on the image plane. Furthermore, actual displacement of the ROI can be extracted when some conditions are met. The proposed technique is validated both in the laboratory using a rigid pipe and in the field on a small pedestrian bridge cable. Results show that the technique is able to measure the pipe motion and the cable vibration accurately. The proposed technique requires only one commercial camera, and no prior camera calibration is needed. In addition, the use of an optical flow method eliminates the need to attach any target to the cable and makes the technique very easy to implement. Despite these advantages, the technique still needs further development before it can be applied to long-span bridge cables.
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paper_title: Extracting modal parameters of a cable on shaky motion pictures

paper_content:

Abstract A set of modal parameters of a cable are extracted from a motion picture captured by a digital camera operated with shaking hands. It is difficult to identify the center of the targets attached to the cable surface from the blurred motion image of the cable, because of the high-speed motion of the cable, low sampling frequency of the camera, and the effect of shaking hands on the motion pictures. This paper proposes a multi-template matching algorithm to solve these difficulties. In addition, a sensitivity-based system identification algorithm is proposed for extracting the natural frequencies and the damping ratios from ambient cable vibration data. Three sets of vibration tests are performed to examine the validity of the proposed algorithms. The results show that the proposed approach of using these two algorithms is fairly feasible for extracting modal parameters from severely blurred motion pictures.
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paper_title: Structural damage detection using digital video imaging technique and wavelet transformation

paper_content:

Damage in structures may render risk of catastrophic failure. Identifying damages and their locations is termed as damage detection. In this paper, use of digital video imaging is proposed for detecting damage in structures. The theory of measuring structural vibration using high-resolution images is presented first, based on sub-pixel edge identification. Then a concept of mode shape difference function is developed for structural damage detection. A laboratory test program was carried out to implement these concepts using a high-speed digital video camera. The images were analyzed to obtain displacement time series at sub-pixel resolution. Mode shapes were obtained from the time series to find the mode shape difference functions between the damaged and the reference states. They were subjected to wavelet transformation for determining the damage locations. Results show that the proposed approach is able to identify the introduced damage cases and their locations.
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paper_title: Measurement of rivulet movement on inclined cables during rain–wind induced vibration

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Abstract The large amplitude vibration of stay cables has been observed in several cable-stayed bridges under the simultaneous occurrence of rain and wind, which is called rain–wind induced vibration (RWIV). During RWIV, the upper rivulet oscillating circumferentially on the inclined cable surface is widely considered to have an important role in this phenomenon. However, the small size of rivulets and high sensitivity to wind flow make the measurement of the rivulet movement challenging. This study proposes a digital image processing method to measure the rivulet movement in wind tunnel tests. RWIV of a cable model was excited during the test and a digital video camera was used to record the video clips of the rivulets, from which the time history of the rivulet movement along the entire cable is identified through image processing. The oscillation amplitude, equilibrium position, and dominant frequency of the upper rivulet are investigated. Results demonstrated that the proposed non-contact, non-intrusive measurement method is cost-effective and has good resolution in measuring the rivulet vibration. Finally the rivulet vibration characteristics were also studied when the cable was fixed. Comparison demonstrates the relation between the upper rivulet and cable vibration.
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paper_title: Damage Detection Using Optical Measurements and Wavelets

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The paper presents an application of the wavelet transform for damage detection based on optical measurements. A number of important issues, that needs to be considered when image sequences are used for vibration analysis, are discussed. These include: correspondence of image features from image to image, image calibration and spatial resolution. The principles of image edge detection are discussed and a comparison between the wavelet approach and the classical method is presented. A novel damage detection method based on optically measured modeshape data is proposed. The method is illustrated using a simple cantilever beam experiment. The major advantage of the method is the significantly increased number of discrete points used to describe modeshapes. This is in contrast to classical techniques where in practice a small number of measurement points are obtained from a limited number of sensors.
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paper_title: A New Position Measurement System Using a Motion-Capture Camera for Wind Tunnel Tests

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Considering the characteristics of wind tunnel tests, a position measurement system that can minimize the effects on the flow of simulated wind must be established. In this study, a motion-capture camera was used to measure the displacement responses of structures in a wind tunnel test, and the applicability of the system was tested. A motion-capture system (MCS) could output 3D coordinates using two-dimensional image coordinates obtained from the camera. Furthermore, this remote sensing system had some flexibility regarding lab installation because of its ability to measure at relatively long distances from the target structures. In this study, we performed wind tunnel tests on a pylon specimen and compared the measured responses of the MCS with the displacements measured with a laser displacement sensor (LDS). The results of the comparison revealed that the time-history displacement measurements from the MCS slightly exceeded those of the LDS. In addition, we confirmed the measuring reliability of the MCS by identifying the dynamic properties (natural frequency, damping ratio, and mode shape) of the test specimen using system identification methods (frequency domain decomposition, FDD). By comparing the mode shape obtained using the aforementioned methods with that obtained using the LDS, we also confirmed that the MCS could construct a more accurate mode shape (bending-deflection mode shape) with the 3D measurements.
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paper_title: Application of modal analysis supported by 3D vision-based measurements

paper_content:

In the paper, applications of the 3D vision techniques to the modal analysis method are presented. The goal of the project was to develop a methodology for vibration amplitude measurements and a software tool for modal analysis based on visual data. For this purpose, dedicated procedures and algorithms based on the vision technique methods were developed. 3D measurements of vibrations and structure geometry were obtained by application and developing passive 3D vision techniques. The amplitude of vibrations was calculated for selected points on the structure. Necessary vision data were received from the high-speed digital camera "X-Stream Vision" in form of "avi" files that were used as the input data for the developed software tool. The amplitude of vibration displacements obtained from vision-based measurements were treated as the input data for operational modal analysis algorithms. In this domain, the Balanced Realization algorithm has been used.
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paper_title: MULTI-POINT MEASUREMENT OF STRUCTURAL VIBRATION USING PATTERN RECOGNITION FROM CAMERA IMAGE

paper_content:

Modal testing requires measuring the vibration of many points, for which an accelerometer, a gab sensor and laser vibrometer are generally used. Conventional modal testing requires mounting of these sensors to all measurement points in order to acquire the signals. However, this can be disadvantageous because it requires considerable measurement time and effort when there are many measurement points. In this paper, we propose a method for modal testing using a camera image. A camera can measure the vibration of many points at the same time. However, this task requires that the measurement points be classified frame by frame. While it is possible to classify the measurement points one by one, this also requires much time. Therefore, we try to classify multiple points using pattern recognition. The feasibility of the proposed method is verified by a beam experiment. The experimental results demonstrate that we can obtain good results.
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paper_title: Experimental methodology for the dynamic analysis of slender structures based on digital image processing techniques

paper_content:

There are several problems in engineering where the structural response may be affected by conventional devices for displacement measurements. This work presents an alternative methodology for the experimental dynamic analysis of structures, which cannot be monitored by conventional sensors. This methodology is based on digital image acquisition and processing techniques. The importance of this methodology is that it enables one to perform non-contact displacement measurements, without introducing undesirable changes in the structure's behavior, as for example, in models with an extremely reduced scale. The proposed methodology uses sticking markers of a negligible mass in relation to the structure, facilitating the implementation of the instrumentation of the experimental test. To evaluate the efficiency, precision and accuracy of this methodology, two different experimental tests are carried out and the obtained results are favorably compared with the theoretical results and with the ones provided by conventional sensors. This has shown to be a highly cost-effective and easy to implement experimental procedure for the analysis of very flexible structures, and yet maintains the advantage of dynamic measurements with high resolution.
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paper_title: Digital image processing for non-linear system identification

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Abstract Emerging digital image processing techniques demonstrate their potential applications in engineering mechanics, particularly in the area of system identification involving non-linear characteristics of mechanical and structural systems. The objective of this study is to demonstrate the proof-of-concept that the techniques permit the identification non-intrusively and remotely. First, the efficacy of the digital image processing method is shown by identifying the friction behavior between two solids that are assumed to be governed by the Coulomb friction model. The inverse analysis is carried out to validate the proposed method of identifying model parameters. Studies further illustrate that the digital imaging procedure and inverse analysis algorithms developed for the friction problem can be extended for identification of non-linear mechanical and structural characteristics. One illustration utilizes the second example, where relative motion between a model structure and a shaking table is measured by digitally processing the analogue image provided by a video tape recorded during a shaking table test of a model structure base isolated by a hybrid isolation device consisting of friction and elastomeric components. Third, constitutive relationship for a non-linear elastomeric membrane is identified by digitally processing images of its deformed states under tension. The relationship is postulated to follow the Mooney–Rivlin function. Schemes developed are verified with iterative non-linear finite-element program that is valid in the finite deformation range. Emerging cost-effective hardware and software systems for high-performance data acquisition and processing are quite promising to the implementation of the techniques by removing most, if not all, of its existing limitations.
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paper_title: Remote nondestructive evaluation technique using infrared thermography for fatigue cracks in steel bridges

paper_content:

Long-standing infrastructure is subject to structural deterioration. In this respect, steel bridges suffer fatigue cracks, which necessitate immediate inspection, structural integrity evaluation or repair. However, the inaccessibility of such structures makes inspection time consuming and labour intensive. Therefore, there is an urgent need for developing high-performance nondestructive evaluation (NDE) methods to assist in effective maintenance of such structures. Recently, use of infrared cameras in nondestructive testing has been attracting increasing interest, as they provide highly efficient remote and wide area measurements. This paper first reviews the current situation of nondestructive inspection techniques used for fatigue crack detection in steel bridges, and then presents remote NDE techniques using infrared thermography developed by the author for fatigue crack detection and structural integrity assessments. Furthermore, results of applying fatigue crack evaluation to a steel bridge using the newly developed NDE techniques are presented.
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paper_title: Automatic concrete health monitoring: assessment and monitoring of concrete surfaces

paper_content:

To predict the degradation of concrete structures is extremely challenging. The typical approach combines periodic visual inspections with required non-destructive tests. However, this methodology only discretely evaluates few areas of the structure, being also time consuming and subject to human error. Therefore, a new method designated ‘automatic concrete health monitoring’ is herein presented which aims at automatically characterising and monitoring the state of conservation of concrete surfaces by combining photogrammetry, image processing and multi-spectral analysis. The method was designed to (i) characterise crack pattern, displacement and strain fields; (ii) map damages and (iii) assess and define restoration tasks.
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paper_title: Image-Based Monitoring of Open Gears of Movable Bridges for Condition Assessment and Maintenance Decision Making

paper_content:

AbstractMovable bridges are unique structures due to the complex interaction between their structural, mechanical, and electrical systems with an intricate interrelation creating several challenges related to operation and maintenance. Continuous monitoring of the critical parts of these structures is essential to track and evaluate their performance for improving maintenance operations and reducing the associated costs. Open gears are one of the most critical components of movable bridges. Proper and regular maintenance of these gears is vitally important to ensure a safe, reliable, and cost-effective operation. In this study, a practical and low-cost monitoring approach is presented to track the lubrication level in an open gear of a movable bridge by using video cameras. Two unique indices are developed for monitoring of the open gear by investigating two different image processing methods in a comparative fashion. The first methodology is based on an edge detection algorithm that utilizes a Sobel grad...
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paper_title: Improvement of Crack-Detection Accuracy Using a Novel Crack Defragmentation Technique in Image-Based Road Assessment

paper_content:

AbstractA common problem of crack-extraction algorithms is that extracted crack image components are usually fragmented in their crack paths. A novel crack-defragmentation technique, MorphLink-C, is proposed to connect crack fragments for road pavement. It consists of two subprocesses, including fragment grouping using the dilation transform and fragment connection using the thinning transform. The proposed fragment connection technique is self-adaptive for different crack types, without involving time-consuming computations of crack orientation, length, and intensity. The proposed MorphLink-C is evaluated using realistic flexible pavement images collected by the Florida Department of Transportation (FDOT). Statistical hypothesis tests are conducted to analyze false positive and negative errors in crack/no-crack classification using an artificial neural network (ANN) classifier associated with feature subset selection methods. The results show that MorphLink-C improves crack-detection accuracy and reduces...
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paper_title: Vision-Based Automated Crack Detection for Bridge Inspection

paper_content:

The visual inspection of bridges demands long inspection time and also makes it difficult to access all areas of the bridge. This paper presents a visual-based crack detection technique for the automatic inspection of bridges. The technique collects images from an aerial camera to identify the presence of damage to the structure. The images are captured without controlling angles or positioning of cameras so there is no need for calibration. This allows the extracting of images of damage sensitive areas from different angles to increase detection of damage and decrease false-positive errors. The images can detect cracks regardless of the size or the possibility of not being visible. The effectiveness of this technique can be used to successfully detect cracks near bolts.
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paper_title: Integrated Vision-Based System for Automated Defect Detection in Sewer Closed Circuit Television Inspection Videos

paper_content:

AbstractThis paper discusses the development of a general framework and software system to support automated analysis of sewer inspection closed-circuit television (CCTV) videos. The proposed system aims primarily to support the off-site review and quality control process of the videos and to enable efficient reevaluation of archived CCTV videos to extract historical sewer condition data. Automated analysis of sewer CCTV videos poses several challenges including the nonuniformity of camera motion and illumination conditions inside the sewer. The paper presents a novel algorithm for optical flow-based camera motion tracking to automatically identify, locate, and extract a limited set of video segments, called regions of interest (ROI), that likely include defects, thus reducing the time and computational requirements needed for video processing. The proposed algorithm attempts to recover the operator actions during the inspection session, which would enable determining the location and relative severity of...
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paper_title: Concrete Crack Assessment Using Digital Image Processing and 3D Scene Reconstruction

paper_content:

AbstractTraditional crack assessment methods for concrete structures are time consuming and produce subjective results. The development of a means for automated assessment employing digital image processing offers high potential for practical implementation. However, two problems in two-dimensional (2D) image processing hinder direct application for crack assessment, as follows: (1) the image used for the digital image processing has to be taken perpendicular to the surface of the concrete structure, and (2) the working distance used in retrieving the imaging model has to be measured each time. To address these problems, this paper proposes a combination of 2D image processing and three-dimensional (3D) scene reconstruction to locate the 3D position of crack edges. In the proposed algorithm, first the precise crack information is obtained from the 2D images after noise elimination and crack detection using image processing techniques. Then, 3D reconstruction is conducted employing several crack images to ...
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paper_title: An efficient image-based damage detection for cable surface in cable-stayed bridges

paper_content:

Abstract Since cable members are the major structural components of cable bridges, they should be properly inspected for surface damage and inside defects such as corrosion and/or breakage of wires. This study introduces an efficient image-based damage detection system that can automatically identify damages to the cable surface through image processing techniques and pattern recognition. The damage detection algorithm combines image enhancement techniques with principal component analysis (PCA) algorithm. Images from three cameras attached to a cable climbing robot are wirelessly transmitted to a server computer located on a stationary cable support. To improve the overall quality of the images, this study utilizes an image enhancement method together with a noise removal technique. Next the input images are projected into PCA sub-space, the Mahalanobis square distance is used to determine the distances between the input images and sample patterns. The smallest distance is found to be a match for an input image. The proposed damage detection algorithm was verified through laboratory tests on three types of cables. Results of the tests showed that the proposed system could be used to detect damage to bridge cables.
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paper_title: Sensor Networks, Computer Imaging, and Unit Influence Lines for Structural Health Monitoring: Case Study for Bridge Load Rating

paper_content:

In this paper, a novel methodology for structural health monitoring of a bridge is presented with implementations for bridge load rating using sensor and video image data from operating traffic. With this methodology, video images are analyzed by means of computer vision techniques to detect and track vehicles crossing the bridge. Traditional sensor data are correlated with computer images to extract unit influence lines (UILs). Based on laboratory studies, UILs can be extracted for a critical section with different vehicles by means of synchronized video and sensor data. The synchronized computer vision and strain measurements can be obtained for bridge load rating under operational traffic. For this, the following are presented: a real life bridge is instrumented and monitored, and the real-life data are processed under a moving load. A detailed finite-element model (FEM) of the bridge is also developed and presented along with the experimental measurements to support the applicability of the approach for load rating using UILs extracted from operating traffic. The load rating of the bridges using operational traffic in real life was validated with the FEM results of the bridge and the simulation of the operational traffic on the bridge. This approach is further proven with different vehicles captured with video and measurements. The UILs are used for load rating by multiplying the UIL vector of the critical section with the load vector from the HL-93 design truck. The load rating based on the UIL is compared with the FEM results and indicates good agreement. With this method, it is possible to extract UILs of bridges under regular traffic and obtain load rating efficiently.
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paper_title: Combined Imaging Technologies for Concrete Bridge Deck Condition Assessment

paper_content:

Evaluating the condition of concrete bridge decks is an increasingly important challenge for transportation agencies and bridge inspection teams. Closing the bridge to traffic, safety, and time consuming data collection are some of the major issues during a visual or in-depth bridge inspection. To date, several nondestructive testing technologies have shown promise in detecting subsurface deteriorations. However, the main challenge is to develop a data acquisition and analysis system to obtain and integrate both surface and subsurface bridge health indicators at higher speeds. Recent developments in imaging technologies for bridge decks and higher-end cameras allow for faster image collection while driving over the bridge deck. This paper will focus on deploying nondestructive imaging technologies such as the three-dimensional (3D) optical bridge evaluation system (3DOBS) and thermal infrared (IR) imagery on a bridge deck to yield both surface and subsurface indicators of condition, respectively. Spall and delamination maps were generated from the optical and thermal IR images. Integration of the maps into ArcGIS, a professional geographic information system (GIS), allowed for a streamlined analysis that included integrating and combining the results of the complimentary technologies. Finally, ground truth information was gathered through coring several locations on a bridge deck to validate the results obtained by nondestructive evaluation. This study confirms the feasibility of combining the bridge inspection results in ArcGIS and provides additional evidence to suggest that thermal infrared imagery provides similar results to chain dragging for bridge inspection.
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paper_title: Strain and Displacement Controls by Fibre Bragg Grating and Digital Image Correlation

paper_content:

Test control is traditionally performed by a feedback signal from a displacement transducer or force gauge positioned inside the actuator of a test machine. For highly compliant test rigs, this is a problem since the response of the rig influences the results. It is therefore beneficial to control the test based on measurements performed directly on the test specimen. In this paper, fibre Bragg grating (FBG) and Digital Image Correlation (DIC) are used to control a test. The FBG sensors offer the possibility of measuring strains inside the specimen, while the DIC system measures strains and displacement on the surface of the specimen. In this paper, a three-point bending test is used to demonstrate the functionality of a control loop, where the FBG and DIC signals are used as control channels. The FBG strain control was capable of controlling the test within an error tolerance of 20 µm m−1. However, the measurement uncertainty offered by the FBG system allowed a tolerance of 8.3 µm m−1. The DIC displacement control proved capable of controlling the displacement within an accuracy of 0.01 mm.
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paper_title: Integration of computer imaging and sensor data for structural health monitoring of bridges

paper_content:

The condition of civil infrastructure systems (CIS) changes over their life cycle for different reasons such as damage, overloading, severe environmental inputs, and ageing due normal continued use. The structural performance often decreases as a result of the change in condition. Objective condition assessment and performance evaluation are challenging activities since they require some type of monitoring to track the response over a period of time. In this paper, integrated use of video images and sensor data in the context of structural health monitoring is demonstrated as promising technologies for the safety of civil structures in general and bridges in particular. First, the challenges and possible solutions to using video images and computer vision techniques for structural health monitoring are presented. Then, the synchronized image and sensing data are analyzed to obtain unit influence line (UIL) as an index for monitoring bridge behavior under identified loading conditions. Subsequently, the UCF 4-span bridge model is used to demonstrate the integration and implementation of imaging devices and traditional sensing technology with UIL for evaluating and tracking the bridge behavior. It is shown that video images and computer vision techniques can be used to detect, classify and track different vehicles with synchronized sensor measurements to establish an input–output relationship to determine the normalized response of the bridge.
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paper_title: Camera-based monitoring of the rigid-body displacement of a mandrel in superconducting cable production

paper_content:

We describe a machine vision measurement head that is used to monitor the mandrel position in production of superconducting cables. Two cameras are orthogonally aligned, viewing different sections of the cylindrical part of the mandrel. The use of telecentric lenses obviates the need for re-calibration after the replacement of mandrel. All parameters of rigid-body motion are obtained in linear theory by using a multivariate least-squares fitting procedure on dynamically corresponded sets of target points that vary due to partial blocking by rotating wires. A rigorous analysis of measurement uncertainty is given.
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paper_title: Vision-based estimation of vertical dynamic loading induced by jumping and bobbing crowds on civil structures

paper_content:

Abstract People's motion on civil structures induces dynamic loading that may lead to excessive vibrations. The complete characterization of this force distribution over a wide area due to a large number of people is still an unsolved issue. This work presents a measuring technique for the vertical load estimation in case of jumping and bobbing crowd, based on the evaluation of the vertical inertia of the human body. Laboratory experiments verify the proposed model on a single volunteer through standard inertial sensors and then extend it introducing the non-contact measuring technique. The method validation is carried out in a real environment: a stand of the G. Meazza stadium in Milan, dynamically characterized in terms of frequency response function. The load induced by groups of jumping people is estimated with the proposed method and the resulting structure accelerations are computed: the comparison between measured and estimated vibrations shows a very high correspondence in both time domain and main spectral components and, above all, the performances do not get worse as the number of volunteer increases.
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paper_title: Study on Three-Dimensional Displacement Measurement Method of EAST Cold Magnet Based on Computer Vision

paper_content:

The location of superconducting tokamak magnets decides the position and shape of plasma, it is significant of real-time acquiring the location of tokamak magnets to stably operate the tokamak. At present, magnet displacement measurement is taken in displacement sensor, but the factors affecting the measurement results have much because of tokamak structural features, this paper proposes an improved monocular laser triangle measuring method, it can effectively reduce the distractions, using the image processing software specially developed, we can quickly get magnet operating data, experimental results show that this method precision is high, this method can meet the requirements of the measure.
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paper_title: Application of computer vision and laser interferometer to the inspection of line scale

paper_content:

In the paper machine vision, laser interferometer and coordinate measuring machine (CMM) are combined to develop a vision inspection system. The measurement capability of the developed system is investigated by measuring the distances between the lines on a standard line scale. The vision camera is used to replace the probe of the CMM to take the images of the interested lines on a line scale at two different positions. Meanwhile, the displacement of the CCD camera is measured using laser interferometer. Using subpixel edge localization and outlier-excluding least-squares regression, the distance between two interested lines is computed under an image plane coordinate system. By adding the displacement of the CCD camera measured using laser interferometer, the line space can be determined. Experiments have been performed repeatedly to measure the line space on the 1.00 and 300.0 mm line scales. Results indicate that the measured data only have a little fluctuation and are close to those obtained by the NML (National Measurement Laboratory, Taiwan).
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paper_title: A new joint application of non-invasive remote sensing techniques for structural health monitoring

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This paper aims at analysing the potentialities of a new technological approach for the dynamic monitoring of civil infrastructures. The proposed approach is based on the joint use of a high-frequency thermal camera and a microwave radar interferometer to measure the oscillations due to traffic excitations of the Sihlhochstrasse Bridge, Switzerland, which was selected as test bed site in the ISTIMES project (EU—Seventh Framework Programme). The good quality of the results encourages the use of the proposed approach for the static and dynamic investigation of structures and infrastructures. Moreover, the remote sensing character of the two applied techniques makes them particularly suitable to study structures located in areas affected by natural hazard phenomena, and also to monitor cultural heritage buildings for which some conventional techniques are considered invasive. Obviously, their reliability needs further experiments and comparisons with standard contact sensors.
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Title: A Review of Machine Vision-Based Structural Health Monitoring: Methodologies and Applications

Section 1: Introduction
Description 1: This section introduces the concept of structural health monitoring (SHM), and discusses the evolution and significance of machine vision-based SHM technologies.

Section 2: Machine Vision Methods
Description 2: This section covers the basic components and procedures involved in machine vision methods for structural monitoring.

Section 3: Image Processing Algorithms
Description 3: This section details various image processing algorithms employed in structural monitoring, explaining their functionalities and applications.

Section 4: Systematic Errors Assessment and Reduction
Description 4: This section addresses different kinds of errors associated with vision-based measurement systems, and describes methodologies for error assessment and reduction.

Section 5: Two-Dimensional (2D) Structural Displacement Monitoring
Description 5: This section focuses on techniques and systems for monitoring 2D structural displacements, including case studies and examples.

Section 6: Three-Dimensional (3D) Structural Displacement Monitoring
Description 6: This section explores methods for 3D structural displacement monitoring using multiple cameras and vision reconstruction techniques.

Section 7: Structural Strain and Stress Monitoring
Description 7: This section discusses vision-based methods to measure structural strain and stress, with examples from various materials and structural elements.

Section 8: Vibration Monitoring and Dynamic Characteristics Identification
Description 8: This section outlines the use of high-speed cameras for vibration monitoring and identification of dynamic characteristics, like natural frequency and modal damping ratio.

Section 9: Crack Inspection and Characterization
Description 9: This section highlights advanced image processing techniques for detecting and characterizing cracks and other surface features on structures.

Section 10: Integration Technology
Description 10: This section describes the integration of machine vision technology with other sensing methods to enhance structural health monitoring capabilities.

Section 11: Conclusions
Description 11: This section provides a summary of the state-of-the-art in machine vision-based SHM, outlining key achievements, limitations, and future prospects in the field.