Non-contact Displacement Measurement Research Articles

Evaluation of Desiccation Behavior in Basalt Microfiber–Reinforced Bentonite Clay for Geological Repositories of Nuclear Spent Fuel Using Digital Image Correlation

Abstract Secure storage of nuclear spent fuel (NSF) is of great concern for protecting public health and safety. The preferred long-term solution is underground containment in geological repositories, where one or more engineered barrier materials (EBM) encapsulate the NSF and separate it from the natural rock. Bentonite clay is commonly used as an EBM due to its many advantageous properties including low hydraulic conductivity, which ensures limitation of water infiltration to the system and the subsequent risk of corrosion in NSF canisters. However, bentonite clay subjected to heating from nuclear decay may form desiccation cracking. This study conducted disk-shaped free shrinkage tests and ring-shaped restrained shrinkage tests of bentonite clay samples reinforced with basalt microfibers. Digital image correlation was used as a noncontact full-field displacement measurement to track the time-evolving shrinkage and desiccation cracking phenomena and make quantified comparisons between plain bentonite and bentonite with varying contents of basalt microfibers (i.e., 0.0, 0.5, 1.0, and 1.5 % wt.). Results indicate that plain bentonite and basalt microfiber-reinforced samples showed similar free shrinkage behavior, while desiccation cracking behavior was significantly altered by adding basalt microfibers. Microfiber reinforcement effectively reduced major cracks through a “crack-bridging” effect while causing minor cracks to initiate earlier and at higher moisture contents than plain bentonite. Results infer that reinforcing plain bentonite with inorganic microfibers can potentially control desiccation cracking, leading to safer and improved nuclear waste management.

Stereovision-based vibration measurement of stay cable using synchronized multi-camera setup and video motion magnification

This paper proposes a new framework for 3D noncontact stereovision-based displacement measurement in conjunction with Phase-based Video Motion Magnification (PVMM) to capture in-plane and out-of-plane vibrations of stay cables. In the proposed method, cable vibrations are captured by a pair of synchronized digital cameras, then PVMM method is applied to magnify micro vibrations of the cable (ambient responses) recorded by video cameras. Next, a camera calibration technique is proposed to convert the 2D image coordinates to 3D global coordinates, followed by the stereo calibration between the two cameras to synchronize the target points. Afterwards, a Centroid-based Bounding-Box Tracking (CBBT) technique is proposed and used to track the movements of the cable in each video frame. As a result, time-history displacements of the cable are extracted from the magnified videos. Finally, natural frequencies and damping ratios of the cable are extracted from the in-plane and out-of-plane displacements using Wavelet-screening Random Decrement Technique (WS-RDT). The proposed methodology has been validated through a couple of wind-tunnel tests of cable vibrations. The results have demonstrated a good agreement between the proposed method and conventional accelerometer-based measurement results.

Mechanical behavior analysis using small-size graphite disc compression testing with digital image correlation methods

The ASTM InternationalStandard Test Method for Tensile Stress-Strain of Carbon and Graphite (ASTM C749) requires dog-bone shaped specimens. Unfortunately, specimen sizes required in ASTM C749 are much too large for irradiation within a test reactor and are incompatible with irradiation capsule volumes, molten salt degradation facilities, or oxidation apparatus. The ASTM InternationalStandard Test Method for Tensile Strength Estimate by Disc Compression of Manufactured Graphite (ASTM D8289) was developed as an alternative that provides a means for testing tensile properties using smaller specimens when geometric constraints are present. However, the important stress–strain relation throughout the test loading history that could be recorded using ASTM C749 is lost by using ASTM D8289 because of the difficulties involved with using traditional strain gauges or extensometers on such a small specimen. In this work, a digital image correlation (DIC) method, which uses a full-field noncontact surface displacement measurement technique, was applied along with the ASTM D8289 splitting tensile test on eight small samples of graphite grade Mersen 2114 to track their deformation during compressive loading. Initial results showed that the DIC technique can measure the surface displacement/strain on these small (Ø6 × 3 mm) graphite specimens with high accuracy and good repeatability, indicating that the DIC technique, combined with the small-disc splitting tensile test in ASTM D8289, can capture the strain evolution in the test. Further comparison of measured strains with analytical and numerical simulation results provides additional understanding of the mechanical behaviour of this nuclear graphite material in complex stress states.

A deformation measurement method based on surface texture information of rocks and its application

Users of the digital image correlation method are faced with the problem of poor operability, low repeatability, and lack of standardized specifications for spraying speckles. To solve the problem, the research proposed a rock deformation measurement method that obviates the need to spray speckles. A local binary model was established by using the local binary pattern (LBP) operator based on deep texture features on rock surfaces. The resulting LBP digital speckle pattern can substitute artificial speckle patterns and demonstrates high quality and strong applicability. Based on the LBP digital speckle pattern, the target tracking algorithm was employed to achieve non-contact measurement of the dynamic displacements of rocks. The feasibility and effectiveness of the algorithm in practical application were verified by conducting shear tests on granite and siltstone. Test results show that the deformation characteristics in the displacement nephograms are in line with the measured data pertaining to rock fracturing and conform to the basic characteristics of the shear failure of rocks. The deformation measurement method based on surface texture information can realize non-contact displacement measurement of rocks under conditions without speckles: this obviates the influence of the quality of sprayed speckles on the accuracy of the measurement of deformation.

A Dual-Beam Differential Method Based on Feedback Interferometry for Noncontact Measurement of Linear and Angular Displacement

A dual-beam differential method for noncontact measurement of linear and angular displacement is reported in this paper. Compared to other optical sensors, the system based on the frequency-modulated feedback interferometry has higher sensitivity for non-cooperative targets and a wider range with respect to the angle measurement. Performance of the proposed method is evaluated via testing a prototype system. The experimental results show that the prototype has a stability of 35 nm and 0.15“ over 1 hour with a resolution of 1 nm and 0.02” respectively. The linearity is 5.58×10^-6 in the range of 100 mm and 1.34×10^-4 in the range of 360°. The experiments for PZT deformation measurement and rotary motor monitor indicate that the proposed method possesses considerable potential for high-precision metrological applications, such as lathe calibration and motor control.

Noncontact Structural Displacement Measurements Using a Low-Cost Motion Tracking System

Structural health monitoring (SHM) depends on precise displacement measurements to evaluate a structure’s condition. Current measurement systems face challenges such as contact-type restrictions, high costs, uniaxial limitations, and significant computational power requirements. A novel low-cost, non-contact displacement measurement system, called the HTC-VTS (Vive Tracking System), developed by Hongda International Electronics Co., Ltd. (New Taipei City, Taiwan), is proposed. The system setup and data acquisition of the HTC-VTS for SHM involve coordinate transformations to convert six degrees of freedom (DOFs) from raw motion data into 3-DOF structural displacement measurements. The HTC-VTS was experimentally validated using laboratory-scale free vibration tests on a cantilever beam and shaking table tests with actual bridge displacement measurements, demonstrating satisfactory performance in both scenarios. The proposed system expands the potential for cost-effective motion capture systems in both experimental and real-world structural applications.

A semi-analytical dynamics method for spindle radial throw in boring process

In precision boring processing, accurate prediction of the radial throw of a boring tool spindle has critical significance for reducing machining errors. However, the classical dynamics method of radial throw based on the rigid body partition method ignores special characteristics of the boring tool spindle, resulting in additional errors regarding radial throw evaluation. Hence, a semi-analytical dynamics method considering the axial length, variable cross-section, and offset distance was proposed to decrease the computational error of the current dynamics model. The mechanism of radial throw was analyzed by the mechanics of materials and mathematical formulas. The Euler-Bernoulli beam theory and partial differential equation method were utilized to derive the dynamics model of the boring tool spindle, and an improved Galerkin variational algorithm was used to resolve the dynamics formulas for the radial throw. Then, the proposed dynamics method was compared with the non-contact displacement measurement and the traditional lumped mass parameter method. The validated results showed that the presented dynamics method had good agreement with the experimental data, and the maximum predictive error was 16.08 μm lower based on the proposed method. Furthermore, the machining precision of holes was also clearly evaluated and discussed using the proposed dynamics method under different processing conditions, and the lower uncertainty of the proposed dynamics method was also demonstrated. As a result, the proposed semi-analytical dynamics method can accurately compute the radial throw and evaluate the machining error in boring technology.

Experimental study on the galloping characteristics of single ice-coated transmission lines under oblique flows

Galloping of ice-coated transmission lines is occasionally observed under oblique wind directions. However, most current investigations on the galloping mechanisms are for flow perpendicular to the span of the transmission lines. In order to address this gap, this research studies the galloping characteristics of ice-coated transmission lines under oblique flows based on wind tunnel tests. The wind-induced displacement of an aero-elastic iced-coated transmission line model was measured with a noncontact displacement measurement equipment in a wind tunnel at different wind speeds and directions. The results show that galloping is characterized by elliptical trajectories and negative damping, which is more likely to occur under oblique flows than the direct flow (0°). At 15° wind direction, a galloping in vertical direction was observed at wind speeds above 5 m/s. At 30° wind direction, galloping was observed over the entire range of the tested wind speeds. Moreover, the galloping amplitudes under oblique flows are observed to be larger than that at the direct flows. Consequently, when the wind direction between the major winter monsoon azimuth and the lateral direction of transmission line route is between 15° and 30°, appropriate anti-galloping devices are highly recommended in practice.

Blade Crack Diagnosis Based on Blade Tip Timing and Convolution Neural Networks

The diagnosis of blade crack faults is critical to ensuring the safety of turbomachinery. Blade tip timing (BTT) is a non-contact vibration displacement measurement technique, which has been extensively studied for blade vibration condition monitoring recently. The fault diagnosis methods based on deep learning can be summarized as studying the internal logical relationship of data, automatically mining features, and intelligently identifying faults. This research proposes a crack fault diagnostic method based on BTT measurement data and convolutional neural networks (CNNs) for the crack fault detection of blades. There are two main aspects: the numerical analysis of the rotating blade crack fault diagnosis and the experimental research in rotating blade crack fault diagnosis. The results show that the method outperforms many other traditional machine learning models in both numerical models and tests for diagnosing the depth and location of blade cracks. The findings of this study contribute to the real-time online crack fault diagnosis of blades.

A non-contact displacement measurement system based on a portable smartphone with digital image methods

Bridges are an indispensable element of modern infrastructure, while the difference in maintenance between the small and medium-sized bridges and large bridges is huge. At present, the health assessments of the small and medium-sized bridges still depend on manual detection, which requires a lot of manpower and material resources with limited efficiency. To overcome these issues, this study proposes a novelty vision-based displacement measurement solution utilizing a smartphone, which includes multiple image recognition methods for different application scenarios and further constructs a complete management and maintenance system. The proposed system is validated in the laboratory tests with a cable-stay bridge and field tests on a continuous plate bridge. Moreover, the field experiments on a continuous girder bridge with far measuring distance under diverse lighting conditions were conducted, investigating the measurement effects of various image methods and providing beneficial explorations for the development of detection and monitoring technology on the small and medium-sized bridges.

Non-contact full-field dynamic strain reconstruction of rotating blades under multi-mode vibration

Dynamic strain measurements of rotating blades have attracted significant attention in the turbomachinery design, testing, and service stages. Measuring the blade full-field strain using traditional strain gauges (SGs) is difficult. Too many SGs may add damping to the measured blades, thereby lowering the peak strain that should have been experienced. The strain level was then underestimated for the blades without SGs. A non-contact displacement measurement technique, called blade tip timing (BTT), is promising for replacing SGs but requires effective displacement-to-strain transformation. The transformation under blade single-mode vibration has been studied in recent years. Although the multi-mode vibration of rotating blades is increasingly common, it is usually ignored. In contrast to single-mode vibration, when the blade is subjected to multi-mode vibration, the peak stress changes in both the temporal and spatial domains. This causes difficulties in predicting the blade stress limit and fatigue life. This study proposes a full-field dynamic strain reconstruction method for rotating blades under multi-mode vibration. Based on the BTT-measured displacement, the proposed method enables the full-field dynamic strain reconstruction of the blade in the time domain. First, the multi-mode displacement response of the rotating blades was measured using the BTT system. Second, a least-squares decoupling model for the multi-mode response was established. The blade tip displacement response was decoupled, and the multi-mode vibration parameters were identified, including the frequency, amplitude, and phase. Third, an analytical transform relationship between the tip displacement and full-field strain was derived based on the blade mode shapes. The blade full-field dynamic strain was reconstructed in the time domain using the decoupled displacement response and a displacement-to-strain transmissibility matrix. Finally, the full-field strain at different instants was visualised using a blade finite element model. The proposed method is validated on a rotor with five blades through both numerical simulations and spinning experiments. The reconstructed strain was compared with the result measured by the SG. The average strain relative errors of the five blades were within 7.1%. The peak stress is predicted, and the critical positions on the blade under multi-mode vibration are marked, which can provide valuable knowledge for blade fatigue life prediction and health monitoring.

LARGE SIZED PRODUCT SHELLS DISPLACEMENT MEASUREMENT UNDER STATIC LOADING USING LASER TRACKERS

In the aerospace industry the accuracy of product geometry and the aerodynamic contour provided traditionally, by the mould loft template method. The assembly accuracy control is carried out using optical measuring instruments, such as a level and a theodolit in combination with leveling rulers came to the aircraft industry from geodesy. Measurements are carried out according to reference points pre-embedded in the product at the design stage.
 The introduction of modern high precision coordinate measuring machines (CMM) into the production cycle is promising and actively developing direction to improve the quality of manufactured aircraft. To control the geometry of parts and assemblies, stationary portal-type CMMs and mobile CMMs of the "hand" type replace traditional rod tools and templates, and in the assembly and control operations of the product, traditional optical devices are replaced by a laser tracker. 
 The aim of this paper is the use of laser trackers for non-contact real-time large-sized product shell displacement measurement under static loading, which makes it possible to reduce the complexity and duration of tests, and ensure high measurement accuracy.

Super-robust digital image correlation based on learning template

Digital image correlation (DIC) is a well-established noncontact displacement measurement method. Nevertheless, there are still three aspects that can be further improved: accuracy, efficiency, and robustness. Compared with accuracy and efficiency, there are few studies on the robustness of DIC method. However, there are many factors that make the DIC algorithm no longer robust, such as cracks, discontinuous displacements, large inter-frame deformation, partial occlusion or contamination, perspective changes, image noise, and illumination variations. This work proposes a learning template based super-robust DIC method which mitigates the influence of all these disadvantage factors in a straightforward and unified approach, significantly improving DIC measurement robustness compared to classic approaches. During the implementation of this method, the online learning and offline learning methods are adopted to keep the correlation at a high level. Interestingly, the learning template can be seen as a deep learning method with only one layer, so this work also explicitly explains why deep learning can be applied to the DIC field. Uniaxial tension tests were conducted and showed that the matching error can meet the accuracy requirements of the initial value. Experiments on landslide images also show that our method significantly improves the matching robustness.

Full-Field Displacement Measurement Method of Metal-As-Insulation Pancake at Liquid Nitrogen Temperature

In situ measurement of metal-as-insulation (MI) high temperature superconductor (HTS) pancake deformation gives insight to its actual mechanical state. Measurement of hoop strain for inner / outer turn of MI pancake is usually made by strain gauges. This measurement is however local and faces electromagnetic compatibility (EMC) and thermal issues. Non-contact full-field displacement measurement is an interesting alternative solution. This technique is based on digital image correlation (DIC), which derives the displacement field in specified region of interest (ROI) by correlation between the deformed state image and the reference state image. In this article, we present a novel DIC experiment setup for displacement measurement of MI pancake in cryogenic cooling. The setup is validated by a cooling test of 304 stainless steel disc. Then, it is used for cooling test of MI pancake made of co-wound copper beryllium - 304 stainless steel tapes. Both tests are done at 80 K by conduction cooling using liquid nitrogen. Finally, experimental results are compared to numerical results obtained from an analytical formulation of the mechanical equilibrium using generalized plane strain assumption.

A Surgeon’s Habits-Based Novel Master Manipulator for the Vascular Interventional Surgical Master-Slave Robotic System

A master-slave vascular interventional surgical robotic system (MSVISRs) promises to treat cardio-cerebrovascular diseases without damaging the health of surgeons. Although many scholars are engaged in the research on the slave manipulator for the MSVISRs, there are very few studies on the master manipulator. In this study, a surgeon’s habits-based novel master manipulator is developed. This master manipulator imitates the operating habits of surgeons in surgery and uses a non-contact method with an optical mouse sensor to measure the operating displacements. The novelty of this approach is that this master manipulator with smaller size and non-contact displacement measurement method can provide the axial force and the circumferential force feedback independently. In this master manipulator, the rigid-flexible coupling structure is used to provide the axial force feedback. In addition, the brush DC motor is used to give the circumferential force feedback. Furthermore, the proposed master manipulator is calibrated and evaluated by experiments. The calibration experiments show that the maximum identification ranges for the axial force and circumferential force in the proposed master manipulator are 0.247-4.5N and 0.52-4.3N, respectively. Moreover, the evaluation experiments show that the maximum error is 0.8 mm in the axial direction. As compared with other master manipulator, the comprehensive properties of this novel master manipulator are shown to be better and more promising.

Using lightweight convolutional neural network to track vibration displacement in rotating body video

The non-contact visual displacement measurement methods can detect problems such as rigid body deformation and structural wear of structural bodies due to long-term service life. However, the traditional visual vibration measurement methods are limited by the inherent sampling constraint of ordinary image sensors, which cannot achieve efficient vibration target identification and correlation tracking for rotating body targets in low-resolution videos under the premise of high sampling rate. Therefore, this paper uses high-speed industrial camera as the acquisition medium, introduces deep convolutional neural networks into the field of visual vibration measurement, on this basis, weighs the target detection accuracy and displacement tracking speed, uses the designed lightweight convolutional neural networks model to vibration displacement tracking in rotating body videos. We ensure that the computational efficiency and parameter quantity of the convolutional networks are solved with low loss accuracy. First of all, we take the lightweight convolutional neural network as the backbone network, replace the standard convolutional neural network with depthwise separable convolution and pointwise convolution. Considering the speed and accuracy advantages of deep learning algorithms for video object tracking, we estimate the heat map, object center offsets and bounding box sizes use an anchor-free detection algorithm on the network framework. In order to prevent the loss of vibrating target identity in rotating body videos, we use re-identification(re-ID) method to strengthen the correlation of target displacement between adjacent frames. In experiments with traditional visual vibration measurement and recent deep learning measurement methods for performance testing, the network model we designed can demonstrate absolute advantages that traditional convolutional neural networks do not have. On the one hand, our comparison of time–frequency characteristics at different speeds shows that the vibration displacement curve regressed by the lightweight convolutional neural network has a high degree of fit with the displacement signal obtained by the eddy current sensor;on the other hand, when the target object is blurred, the generalization ability of our algorithm is proved, which also reflects the engineering application value of visual vibration measurement in the field of vibration displacement tracking of rotating bodies.

Noncontact optical displacement measurements by dynamic contrast auto focusing for slow oscillatory motion

A noncontact displacement system suitable for tracking slow moving surfaces with low reflectivity is demonstrated. The displacement is measured by dynamic tracking of the moving focal plane of the sample under test. The system comprises a camera, a vertical digital piezo driver, and a data acquisition module. The tracking effect is achieved by continuously driving the sample with a 2-μm sweep around the focal plane, simultaneously acquiring the sample position and images of the part of the sample defined as a region of interest (ROI). The position of the focal plane is identified by fitting the contrast of the ROI versus the stage position to a Gaussian function. Using an ROI provides the ability to test various regions across the object and eliminates the demand for the surface to be flat or reflective. The system is low cost, is applicable to a large variety of samples and has an accuracy better than 10 nm.

Dynamic displacement measurement in digital holographic interferometry using eigenspace analysis.

Non-contact measurement of displacement undergone by a deformed object is an important application problem in digital holographic interferometry. Such measurements usually demand reliable estimation of interference phase even in the presence of severe noise. This article describes a method for non-contact displacement testing by investigating a robust phase retrieval approach in digital holographic interferometry. The approach is based on eigenspace processing of the complex interference field signal in digital holographic interferometry. The performance of the proposed method for phase retrieval under severe noise conditions is illustrated using simulation results. The practical utility of the proposed method is demonstrated for dynamic deformation analysis using experimental data from digital holographic interferometry.

Application of double arrowhead auxetic honeycomb structure in displacement measurement

It is essential to measure the amount of displacement in mechanical systems and architectural structures. Researchers have developed displacement sensors using various materials and fabrication methods. However, the components of displacement sensors are in contact with each other, which would cause the interference and noise transmission between the sensing device and conversion element, and also bring much inconvenience to the design and preparation of the sensor. In this study, a non-contact displacement measurement method based on the auxetic structure was developed. The scheme of this method consists of a light source, an auxetic structure and a solar cell. These three parts are not in contact with each other. By this method, the change in transmittance of the auxetic structure due to elastic deformation is transformed into a change in the output current of the solar cell. The proposed method has potential applicability for measuring and monitoring structural parts. The effectiveness of this method is investigated by geometrical analysis and experiments. The results show that the input and output of the proposed methods have good sensitivity. This study provides a new structural view for the displacement sensor and puts forward the future research.

Non-contact, automated adjustment procedure for capacitive displacement sensors

Capacitive sensors enable non-contact displacement measurements in the micrometer range with sub-nanometer resolution. To achieve this, a proper alignment of capacitive sensors is necessary. In this work, a non-contact, automated and intrinsic adjustment procedure is presented which is based on determining the extrema of slope error and nonlinearity. After the final adjustment step, the remaining tilt is less than 1.5 mrad. A measurement routine is used to evaluate the adjustment procedure by applying tilt around the final adjustment position and measuring the global slope error and the global nonlinearity. The combination of adjustment procedure and measurement routine enables the investigation of capacitive displacement sensors with higher accuracy in order to establish a comprehensive uncertainty budget and to explore improvements in capacitive sensor technology.