3D Displacement Measurement Research Articles

Calibration error for dual-camera digital image correlation at microscale

Digital image correlation (DIC) has been widely conducted in many engineering applications. This paper describes a dual-camera system which is mounted on a stereo light microscope to achieve 3D displacement measurement at microscale. A glass plate etched with precision grids was used as the calibration plate and a translation calibration procedure was introduced to obtain the intrinsic and extrinsic parameters of the cameras as well as the aberration of the imaging system. Two main error sources, including grid positioning and stage translation, were discussed. It was found that the subpixel positioning errors had limited influences on displacement measurement, while the incorrect grid positioning can be avoided by analyzing the standard deviation between the grid spacing. The systematic translation error of the stage must be eliminated to achieve accurate displacement measurement. Based on the above analysis, a precisely controlled motorized calibration stage was developed to fulfill fully automatic calibration for the microscopic dual-camera system. An application for measuring the surface texture of the human incisor has been presented. It is concluded that the microscopic dual-camera system is an economic, precise system for 3D profilometry and deformation measurement.

Stroboscopic Fringe Projection Method for 3D Dynamic Displacement Measurement

The determination of dynamic characteristics of microelectromechanical system (MEMS) devices is of great importance. Currently, vibrometer techniques using a Laser-Doppler Vibrometer (LDV) are used for dynamic measurement of MEMS, utilizing an interferometer based on a stroboscope and high-speed cine photomicrography are used for MEMS. But, these methods can’t be used for 3D dynamic displacement measurement simultaneously because of their limitations. In this paper, an optical system for 3D dynamic displacement measurement of micro-components is presented using stroboscopic fringe projection and digital image correlation (DIC), which can measure both in-plane and out-of-plane motions simultaneously. In the system, stroboscopic fringe patterns are projected onto the surface of a vibrating specimen by a phase-shifting projector and stroboscopic illumination supplied with a pulsed laser diode. Synchronization between the stroboscopic laser and the driving signal for the specimen vibrating is achieved by the stroboscopic controller. For a certain vibration state, four deformed phase-shifting fringe patterns are captured by a high-resolution CMOS camera with a long working distance microscope. The images are processed by a phase-shifting technique to obtain the phase distribution. The surface pattern of the specimen without fringes could be obtained by certain phase-shifting algorithms. When stroboscopic pulses are delayed, the stroboscopic phase is changed and another vibrating status could be captured in the same way. Comparing the phase distributions of these two states, the out-of-plane displacement is achieved, which is the displacement of the specimen between these two states. The in-plane displacement could be obtained from the surface pattern without fringes by DIC. Adjusting the phase delay of illumination by stroboscopic controller, the motions of the specimen in the whole vibration period can be obtained.

Two-dimensional (2D) displacement measurement of moving objects using a new MEMS binocular vision system

Displacement measurement of moving objects is one of the most important issues in the field of computer vision. This paper introduces a new binocular vision system (BVS) based on micro-electro-mechanical system (MEMS) technology. The eyes of the system are two microlenses fabricated on a substrate by MEMS technology. The imaging results of two microlenses are collected by one complementary metal–oxide–semiconductor (CMOS) array. An algorithm is developed for computing the displacement. Experimental results show that as long as the object is moving in two-dimensional (2D) space, the system can effectively estimate the 2D displacement without camera calibration. It is also shown that the average error of the displacement measurement is about 3.5% at different object distances ranging from 10 cm to 35 cm. Because of its low cost, small size and simple setting, this new method is particularly suitable for 2D displacement measurement applications such as vision-based electronics assembly and biomedical cell culture.

Experimental Characterisation of Structured Sheet Metal

Structured sheet metals with regular bumps offer higher bending stiffness compared to flat sheet metals. The application of structured sheet metals requires new investigations regarding their strength and deformation behaviour. Standardised testing methods like the tensile test considering defined specimen geometry and measuring methods exist. Those methods, however, have been developed for plain sheets and cannot be directly transferred to structured sheet metals. The assessment of the strength and deformation behaviour of structured sheet metals needs adapted specimen geometry and measuring methods. In this paper the adaption of the standardised tensile test in accordance with DIN EN ISO 6892-1 to the specific characteristics of structured sheet metals is introduced. In order to determine the appropriate specimen geometry their dimensions were methodically varied and the influence of the structure position on the strength and the deformation behaviour was identified. The analysis of the local strain behaviour was carried out by 3D displacement measurement with the ARAMIS-system. For the derivation of the material properties different analysing methods were developed. The test results were compared to those of flat sheet metals.

Multiple-Camera Instrumentation of a Single Point Incremental Forming Process Pilot for Shape and 3D Displacement Measurements: Methodology and Results

A multiple-camera system (more than two cameras) has been developed to measure the shape variations and the 3D displacement field of a sheet metal part during a Single Point Incremental Forming (SPIF) operation. The modeling of the multiple-camera system and the calibration procedure to determine its parameters are described. The sequence of images taken during the forming operation is processed using a multiple-view Digital Image Correlation (DIC) method and the 3D reconstruction of the part shape is obtained using a Sparse Bundle Adjustment (SBA) method. Two experiments that demonstrate the potentiality of the method are described.

Digital holographic interferometer using simultaneously three lasers and a single monochrome sensor for 3D displacement measurements

The use of digital holographic interferometry for 3D measurements using simultaneously three illumination directions was demonstrated by Saucedo et al. (Optics Express 14(4) 2006). The technique records two consecutive images where each one contains three holograms in it, e.g., one before the deformation and one after the deformation. A short coherence length laser must be used to obtain the simultaneous 3D information from the same laser source. In this manuscript we present an extension of this technique now illuminating simultaneously with three different lasers at 458, 532 and 633 nm, and using only one high resolution monochrome CMOS sensor. This new configuration gives the opportunity to use long coherence length lasers allowing the measurement of large object areas. A series of digital holographic interferograms are recorded and the information corresponding to each laser is isolated in the Fourier spectral domain where the corresponding phase difference is calculated. Experimental results render the orthogonal displacement components u, v and w during a simple load deformation.

Shell-specific Interpolation of Measured 3D Displacements, for Micromechanics-Based Rapid Safety Assessment of Shotcrete Tunnels

Shell-specific Interpolation of Measured 3D Displacements, for Micromechanics-Based Rapid Safety Assessment of Shotcrete Tunnels

Front and Side View Image Correlation Measurements on FRP to Concrete Pull-Off Bond Tests

Understanding the transfer of force by bond between externally bonded fiber-reinforced polymer (FRP) reinforcement and concrete is an important step in formulating good models for predicting debonding failures observed in externally bonded reinforcement strengthened systems. In this paper, a 3D optical displacement measurement system was used to capture the full-field displacements from the front and side view in pull-off bond specimens. The experiments were carried using six specimens with carbon FRP (CFRP) strips having different axial stiffnesses but a constant bond length to the concrete substrate. Using the optical measurements, it was possible to obtain the in-plane displacement or slip and the out-of-plane displacement or separation between the CFRP strip and the concrete. It was demonstrated, that the usual assumption of pure shear stresses in such pull-off tests is not true and that the bond behavior is a two-dimensional problem involving shear and peeling stresses. The bond behavior in CFRP strip to concrete pull-off tests was characterized by three stages: (1) the initiation of the first crack; (2) the initiation of debonding; and (3) failure by complete debonding. Based on the test results it was found that there was a dependency between the maximum bond shear stress, the maximum fracture energy of the FRP-concrete interface, and the stiffness of the FRP. However, the slip values after initiation of debonding (Stage 2) were independent of the FRP stiffness. The measured anchorage force and anchorage length were in good agreement with predictions from existing code equations.

Microstructure-Aided Digital Volume Correlation

This paper describes how volumic images of a composite material could be used to measure 3D displacement fields, and assess mechanical properties. The exemple of a tensile test performed on a sample made of an energetic material is chosen. Different tools are used, namely, X-ray microtomography of an in situ experiment, image acquisi- tion and processing, volume correlation to measure 3D displacement fields. The principle of an integrated approach is finally introduced.

Experimental analysis of mechanical response of stabilized occipitocervical junction by 3D mark tracking technique

This study is about a biomechanical comparison of some stabilization solutions for the occipitocervical junction. Four kinds of occipito-cervical fixations are analysed in this work: lateral plates fixed by two kinds of screws, lateral plates fixed by hooks and median plate. To study mechanical rigidity of each one, tests have been performed on human skulls by applying loadings and by studying mechanical response of fixations and bone. For this experimental analysis, a specific setup has been developed to impose a load corresponding to the flexion-extension physiological movements. 3D mark tracking technique is employed to measure 3D displacement fields on the bone and on the fixations. Observations of displacement evolution on the bone according to the fixation show different rigidities given by each solution.

Three-Dimensional Structural Translation and Rotation Measurement Using Monocular Videogrammetry

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.

Three dimensional experimental and numerical multiscale analysis of a fatigue crack

A full three dimensional study of a fatigue crack in cast iron is presented. This analysis involves combining various tools, namely, Synchrotron X-ray microtomography of an in situ experiment, image acquisition and treatment, 3D volume correlation to measure 3D displacement fields, extraction of the crack geometry, eXtended Digital Volume Correlation to account for the crack displacement discontinuity, crack modeling in an elastic material exploiting the actual crack geometry, and finally estimation of stress intensity factors. All these different tasks are based on specific multiscale approaches.

A two-dimensional atom encoder using one lateral-dithered scanning tunneling microscope (STM) tip and a regular crystalline lattice

In this paper, we propose a new 2D displacement atom encoder using only one lateral-dithered scanning tunneling microscope (STM) tip and one mono-crystalline area of the highly oriented pyrolytic graphite (HOPG) crystal. A high-speed lateral circular dither modulation with proper amplitude is applied to a tip scanner to position the STM tip on six specific points on the HOPG crystalline surface. Multi-tunneling-current signals obtained from the six specific points are utilized to determine the lateral 2D displacement based on the two unit lattice vectors on the HOPG crystalline surface. In addition, the interpolation method can be used in the newly proposed method to measure displacement less than the lattice spacing. In the paper, instrumentation and experiments of the new 2D displacement measurement technique are discussed. The experimental results show that the proposed technique has the capability to measure 2D displacement with a resolution of 10 pm order.

From micron-sized needle-shaped hydrates to meter-sized shotcrete tunnel shells: micromechanical upscaling of stiffness and strength of hydrating shotcrete

Knowledge on the stresses in shotcrete tunnel shells is of great importance, as to assess their safety against severe cracking or failure. Estimation of these stresses from 3D optical displacement measurements requires shotcrete material models, which may preferentially consider variations in the water–cement and aggregate–cement ratios. Therefore, we employ two representative volume elements within a continuum micromechanics framework: the first one relates to cement paste (with a spherical material phase representing cement clinker grains, needle-shaped hydrate phases with isotropically distributed spatial orientations, a spherical water phase, and a spherical air phase; all being in mutual contact), and the second one relates to shotcrete (with phases representing cement paste and aggregates, whereby aggregate inclusions are embedded into a matrix made up by cement paste). Elasticity homogenization follows self-consistent schemes (at the cement paste level) and Mori–Tanaka estimates (at the shotcrete level), and stress peaks in the hydrates related to quasi-brittle material failure are estimated by second-order phase averages derived from the RVE-related elastic energy. The latter permits upscaling from the hydrate strength to the shotcrete strength. Experimental data from resonant frequency tests, ultrasonics tests, adiabatic tests, uniaxial compression tests, and nanoindentation tests suggest that shotcrete elasticity and strength can be reasonably predicted from mixture- and hydration-independent elastic properties of aggregates, clinker, hydrates, water, and air, and from strength properties of hydrates. At the structural level, the micromechanics model, when combined with 3D displacement measurements, predicts that a decrease of the water–cement ratio increases the safety of the shotcrete tunnel shell.

Application of Three-Dimensional Displacement and Strain Distribution Measurement by Windowed Phase-Shifting Digital Holographic Interferometry

In order to measure 3D displacement components and surface strain distributions of objects, three systems for digital holographic interferometry were developed. In this study, these three systems are introduced and the applications to measurement of 3D displacement components and surface strain distributions of cantilevers are shown.

Improved Accuracy of 2-D Non-Contact Range Sensors Using 2-D Lateral Effect Position Sensitive Detector

This paper presents a new set of estimation formulae for non-contact 2D displacement measurement by using the principle of triangulation and the position sensitive detector (PSD) as a 2-D range sensor. It has been shown in theory that the new set of formulae outperforms the conventional formulae by providing much more accurate estimate of the 2D displacement. The theoretical analysis has been verified by simulation studies.

3D displacement measurement by white light digital image analysis in frequency domination area

In this paper, white light digital image analysis in frequency domination area for measuring 3D displacement is put forward. The measuring system has the characteristics of whole-field, non-contact and omni-bearing measurement. It is simple and the coherent light is not demanded. Gray images before and after deformation are recorded using two CCDs at two different shooting angles, then digitalized and analyzed by frequency domination correlation arithmetic. The 2D displacement obtained is used to formulate 3D displacement via mathematical transformation. The experimental plate is 40 mm in length, 30 mm in width and 10 mm in height. The definite displacement is given using a 3D precise adjusting frame. The 3D displacement is calculated and compared with the definite displacement. The error is 4%–11% and the availability of this method was certified. Using this method, the compression deformation of a thin-wall cylinder with 140 mm in height, 64 mm in inner diameter and 64.6 mm in external diameter, was also measured. This method is proper to measure dynamic deformation.

Analysis of the failure mechanisms of a weak rock through photogrammetrical measurements by 2D and 3D visions

This research has been dedicated to the experimental study of two different types of marls through uniaxial compressive tests on prismatic specimens. Induced deformations were measured during testing by classical extensometers and by means of photogrammetrical measurements. Either one or two cameras were employed to allow, respectively, 2D or 3D (using a stereoscopic vision) displacement measurements. A specific software was developed and applied to determine displacement and deformation maps at each photogram on the whole specimen surface. The interpretation of experimental tests, carried out for a better understanding of failure phase in weak rock before crack propagation starts, is illustrated in the paper.

A New Approach for Health Monitoring of Structures: Terrestrial Laser Scanning

Abstract: A new approach is presented for health monitoring of structures using terrestrial laser scanning (TLS). Three‐dimensional (3D) coordinates of a target structure acquired using TLS can have maximum errors of about 10 mm, which is insufficient for the purpose of health monitoring of structures. A displacement measurement model is presented to improve the accuracy of the measurement. The model is tested experimentally on a simply supported steel beam. Measurements were made using three different techniques: (1) linear variable displacement transducers (LVDTs), (2) electric strain gages, and (3) a long gage fiber optic sensor. The maximum deflections estimated by the TLS model are less than 1 mm and within 1.6% of those measured directly by LVDT. Although GPS methods allow measurement of displacements only at the GPS receiver antenna location, the proposed TLS method allows measurement of the entire building's or bridge's deformed shape, and thus a realistic solution for monitoring structures at both structure and member level. Furthermore, it can be used to create a 3D finite element model of a structural member or the entire structure at any instance of time automatically. Through periodic measurements of deformations of a structure or a structural member and performing inverse structural analyses with the measured 3D displacements, the health of the structure can be monitored continuously.

Three-dimensional displacement measurement by using reversed phase-shifting electronic speckle pattern interferometry

A method for three-dimensional (3D) displacement measurement by separating out-of-plane displacement from in-plane displacement is presented. A reference beam is added to a dual-beam symmetric illumination electronic speckle pattern interferometry (ESPI) system and shared by the two illuminations. The test object is illuminated by the two object beams, respectively. Two phase maps, which include out-of-plane and in-plane displacement, can be obtained by phase-shifting techniques. In order to decrease electronic noises in the phase maps, one of the phase maps is calculated by the reversed phase-shifting method presented. By using inverse phase distribution, out-of-plane displacement can be easily separated from in-plane displacement by subtraction and can greatly decrease electronic noises. The principle of the method is presented and proved by a typical three-point bending experiment. Experimental results are offered.