Accurate deflection measurement is vital in evaluating the structural integrity of transportation infrastructures, with bridges being fascinating. In this study, we propose a novel image stabilization technique integrated into the sampling moiré method, which leads to a dependable approach for measuring bridge deflection through drone aerial photography. Our experimental verification entailed conducting drone tests on an actual bridge, utilizing a passing test vehicle, and the results showcased deflection measurements comparable to those obtained through conventional methods. This newly developed technology eliminates the need for ground-fixed cameras mounted on tripods, thus enabling precise deflection measurement at the millimeter level for bridges in challenging environments, including marine and mountainous areas.
Social infrastructures are rapidly aging, and there are concerns about the increasing cost and effort required for maintenance and management. Deflection measurement is critical in evaluating the integrity of bridges as transportation infrastructure. The sampling moiré method was developed to accurately measure the displacement of structures by capturing the regular patterns (i.e., moiré markers) attached to the structures with a digital camera. The conventional approach rigidly attaches a camera to a tripod or a fixed point. However, finding a place to photograph bridges over the sea or mountains can be challenging in real scenarios. In recent years, camera-equipped drones have rapidly become an inspection technology for bridges and other transportation infrastructure. Here, we believe drone cameras open a new door for unconstrained bridge vision inspection using the moiré phase analysis method. Therefore, we are striving to develop a novel displacement measurement method that can measure the deflection of bridges by drone aerial photography. We measured the vertical displacement of a target with a 50-mm-pitch grid pattern in the laboratory to verify its effectiveness. The newly developed measurement technology alleviates the restriction that the camera must be fixed and enables the measurement of the deflection of a more significant number of bridges under various real situations.
In recent times, both the windowed Fourier ridges (WFR) and sampling moiré (SM) algorithms have been extensively used due to their high effectiveness in the demodulation of carrier fringe patterns. As they are developed independently, they are mostly recognized as completely different techniques, but we theoretically prove that SM is a special WFR with a specific window shape and a preset local frequency. This unifies the two different algorithms and enhances the understanding of their theoretical aspects, which helps to simplify the selection of these algorithms in real applications.
In this study, the influence of the working distance (WD) on strain measurement under a laser scanning microscope and a way to achieve precise focus were investigated by the scanning moiré method. Experimental results showed that the strain measurement has a good repeatability at a fixed WD. Scanning moiré fringes were clearly observable when the WD variation range was within 0.9% of the given WD of the used objective lens. The relationship of the measured strain error and the WD difference was approximately linear, and the greatest strain error was near 700 με. Fortunately, 2D moiré fringes were distinct only in a very narrow range, i.e., the WD difference was less than 0.1% of the given WD, and the greatest strain error was less than 100 με. 1D moiré fringes in the y direction, 2D moiré fringes in the both x and y directions, and 1D moiré fringes in the x direction became distinct alternately along with the WD change. Consequently, we suggest to use 2D moiré fringes for microscale strain measurement in each focusing process to reduce the errors caused by the WD variation. Moreover, a single-shot 2D moiré image is useful to measure the strain distributions in both two directions quickly and simply, and there is no need to rotate the sample or scanning lines and scan twice as in the conventional way.
Phase analysis techniques of fringe patterns have been widely used for noncontact three-dimensional shape and deformation measurement by the fringe projection method. Recently, we developed two novel accurate phase analysis methods. One is the two-dimensional sampling moiré method to perform robust phase analysis for a single-shot fringe pattern. The other is the two-dimensional spatiotemporal phase-shifting method to analyze phase distribution accurately for multi-step phase-shifted fringe patterns. To perform accurate phase analysis under low signal-to-noise ratio conditions, both the above two methods use the two-dimensional discrete Fourier transform or fast Fourier transform. Therefore, these algorithms are computationally expensive compared with the conventional one-dimensional sampling moiré and phase-shifting methods. In this study, a fast parallelization implementation for two-dimensional phase-shifting methods, including the two-dimensional sampling moiré method and the spatiotemporal phase-shifting method, are presented by utilizing multi-core CPU. Simulation and experimental results demonstrate that phase analysis can reach 7.5 and 5.9 times faster by use of a 12-core CPU compared with a single CPU.
The deformation distributions of carbon fiber reinforced plastics (CFRP) under a three-point bending load were nondestructively investigated using the phase shifting scanning electron microscope (SEM) moiré method. The complex fast Fourier transform (FFT) and the discrete Fourier transform (DFT) were used to filter the useless moiré fringes in the case of bidirectional moiré fringes. The SEM moiré fringes under different magnifications and the deformation results measured by the direct, complex FFT- and the DFT- phase shifting moiré methods as well as the moiré fringe centering method were compared and analyzed. Experiments demonstrate that the deformation measurement is a bit influenced by the useless moiré fringes in the phase shifting moiré methods and complex FFT processing works better for nondense moiré fringes. The relative strain changes gradually and the specimen grating pitch increases gradually from top to bottom along the loading direction, suggesting that the real compressive strain is greater in the upper side. The micro/nano-scale deformation distribution characteristic is helpful for better understanding of the mechanical properties of the CFRP specimen.
Optical methods providing full-field deformation data have potentially enormous interest for mechanical engineers. In this study, an in-plane and out-of-plane displacement measurement method based on a dual-camera imaging system is proposed. The in-plane and out-of-plane displacements are determined simultaneously using two measured in-plane displacement data observed from two digital cameras at different view angles. The fundamental measurement principle and experimental results of accuracy confirmation are presented. In addition, we applied this method to the displacement measurement in a static loading and bending test of a solid rocket motor case (CFRP material; 2.2 m diameter and 2.3 m long) for an up-to-date Epsilon rocket developed by JAXA. The effectiveness and measurement accuracy is confirmed by comparing with conventional displacement sensor. This method could be useful to diagnose the reliability of large-scale space structures in the rocket development.
Phase analysis plays a role in optical science and technology. For instance, phase analysis technique has been widely used for 3-D shape and deformation measurement by fringe projection profilometry. To analyze the phase distribution of a single fringe pattern, various fringe pattern analysis methods such as a Fourier transform, a wavelet transform, and the windowed Fourier transform have been developed. In this study, a fast phase analysis technique, i.e., two-dimensional sampling moiré method, is proposed to determine accurately the phase distribution of a single fringe pattern by using two-dimensional intensity information. In this method, we record diagonally a single fringe pattern image by using a CCD camera, and perform the image processing of down-sampling with a sampling pitch and intensity interpolation in both x- and y-directions to generate a two-dimensional phase-shifted moiré fringe. Then, the phase distribution of the moiré fringe can be determined by using phase-shifting method and a two-dimensional discrete Fourier transform (DFT) algorithm. Finally, the desired phase distribution of the original fringe pattern can be obtained by adding the phase of the sampling point to the phase of the moiré pattern. By the proposed method, the phase error caused by the random noise of the camera can be dramatically decreased because the intensity information is much richer than one-dimensional intensity data, which utilizes a two-dimensional DFT algorithm. The fundamental principle and primary simulation and experimental results are presented. Theses results show that phase analysis can be performed under extremely low signal-to-noise ratio measurement condition.
Imaging based nondestructive monitoring systems are critical for evaluation of large-scale infrastructures. In this study, an accurate and fast in-plane displacement measurement method based imaging technique is developed for the purpose of health monitoring of large-scale infrastructures such as high building, long bridge, etc. The build-in repeated patterns on infrastructure facade, such as tile, checker, and brick wall pattern is used to measure the in-plane displacement distribution accurately. By performing down-sampling and intensity interpolation image processing to the images captured before and after deformations, multiple phase-shifted moiré fringe can be obtained simultaneously. The phase distribution of the moiré fringe is calculated using the phase shifting method and discrete Fourier transform technique. In the present study, both the fundamental and high frequency components are considered to analyze the repeated patterns. The in-plane displacement distribution can be obtained from the phase differences of the moiré fringe before and after deformations. Compared with conventional displacement methods and sensors, the main advantages of the method developed herein are high-resolution, accurate, fast, low-cost, and easy to implement. The principle of the proposed inplane displacement measurement is presented. The effectiveness of our method is confirmed by a simple displacement measurement experiment. Experimental result showed that a sub-millimeter displacement could be successfully detected for the field of view with meter-scale.
Fringe projection methods using the phase-shifting technique have the advantages of fast 3-D shape measurement and high accuracy. The performance of the fringe projection system, including image quality, nonlinearity of the projected intensity, stability, and switching time for multiple phase-shifted patterns, is essential for fast and accurate shape measurement. A fast and accurate measurement system using a ferroelectric liquid-crystal-on-silicon microdisplay and a high-powered light emitting diode light source is developed. Our results indicate that the nonlinearity of the projected intensity and the stability of the fringe projection were dramatically improved compared with an ordinary commercial liquid crystal display projector. The rapid measurement of the warpage distribution of a flip chip ball grid array electronic package was performed by using the developed system. Nine phase-shifted fringe images with a resolution of 1280×960 pixels were recorded in 1.6 s. In addition, the measurement results obtained by our system agreed well with the results obtained from a micrometer and laser focus sensor. The average error was 2.6 µm, and the standard deviation was less than 10 µm with a 6-mm measurement range.
We present a new single-shot 3-D shape measurement method using a digital micromirror device (DMD) camera and fringe projection. In optics of the DMD camera, individual DMD mirrors operate as controllable high-speed shutters for corresponding charge-coupled device (CCD) pixels. Therefore, four phase-shifted images can be recorded within one frame of the CCD camera by integrated phase-shifting methods using correlations. To obtain 3-D information of the object, an easy and accurate phase-to-3-D calibration method is performed. The principle and hardware are presented. The experimental results under dynamic conditions show that 3-D shape information can be analyzed from only a single image.
DMD (Digital Micro-mirror Device) is a new device, which has hundreds of thousands of micro-mirrors in one chip. This paper presents results of the development of a camera system based on DMD technology for phase analysis and shape measurement that we call "DMD reflection-type CCD camera" or "DMD camera". Incorporation of DMD technology enables accurate control of the intensity reaching the imaging detector of a camera. In order to perform accurate pixel-to-pixel correspondence adjustment with high accuracy, we use a moire technique. In addition, we introduce a high-speed controllable DMD operation board and improve the software to control each DMD mirror with high-speed. As the results, each DMD mirror works as a high-speed controllable shutter for the corresponding CCD pixel. Furthermore, as an application using the DMD camera, we perform an experiment by "DMD-type integrated phase-shifting method using correlations," which can analyze the phase distributions of projected grating from one image taken by the DMD camera. These principles and experimental results in dynamic condition are shown.
Non-contacting shape measurement for 3-D objects is important in automated manufacturing, quality control of components, 3-D solid modeling, etc. Optical measurement of omnidirectional shape has been done by rotating an object and/or measuring it from different directions. We previously proposed a phase-shifting method using Fourier transform (PSM/FT) and a multi-reference-planes method (MRPM) to obtain geometric parameters without influence of lens distortions. Both a measured object and a reference object are simultaneously measured from different directions. All partial point-clouds can be merged into one global coordinate system by a transform matrix calculated from the reference column on a rotary stage. 360-deg 3-D shape can be measured using the above method.
In the PSM/FT, since the initial phase information is determined from only the first frequency of the Fourier spectrum of the phase-shifted intensity values at each point of an object and the frequency components higher than the first frequency almost depend on noise, almost experimental noise can eliminated. The phase reliability evaluation value using Fourier transform (PREV/FT) is, therefore, defined as the ratio of the first frequency component of the Fourier spectrum to the average of the frequency components higher then the first frequency of the Fourier spectrum. The PREV/FT is useful to merge data when measurement conditions are changed. In this paper, we propose a method that all partial data can be merged into global coordinates using the PREV/FT on overlapped areas and omnidirectional shape measurement is achieved.
We propose an evaluation method for the reliability of the analyzed phase in a phase-shifting method using a Fourier transform (PSM/FT) for shape measurement by fringe projection method. In PSM/FT, since the initial phase information is determined only from the first frequency of the Fourier spectrum of the phase-shifted intensity values at each point of an object, and the frequency components higher than the first frequency almost depend on noise, experimental noise can almost be eliminated. The phase reliability evaluation value using a Fourier transform (PREV/FT) is, therefore, defined as the ratio of the first frequency component of the Fourier spectrum to the average of the frequency components higher than the first frequency of the Fourier spectrum. PREV/FT is useful to merge data when we change the measurement condition for shape measurement. We apply the PREV/FT to eliminate the halation effect on a metallic object. Shape measurement of an aluminum specimen that is a rotational body produced by a lathe is performed as an experiment. The results show that the halation effect is eliminated successfully by selecting the highest PREV/FT.
We previously developed a DMD(Digital Micro-mirror Device) reflection-type CCD camera that we call 'DMD camera' and we applied the DMD camera to shape measurement. In this former method, the integral values at a point are obtained by the averaging with the surrounding 9 pixels. This method, therefore, causes a problem that errors are caused by misalignment between the CCD pixel and the DMD pixel. In this paper, we propose a method to improve the algorithm to obtain the phase of a projected grating by phase-shifting method using correlation. The transferred on/off pattern to the DMD are modified and the integral values at a point are obtained by the averaging with the vertical 4 pixels. The accuracy of the shape measurement is improved using the proposed method. The principle, an experimental result, and a comparison of the accuracy between the former method and the proposed method are shown.
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