Non-contact Measurement Technique Research Articles

Blade tip timing for multi-mode identification based on the blade vibration velocity

Blade Tip Timing (BTT) is a widely employed non-contact measurement technique for monitoring turbomachinery blade vibrations. The conventional BTT method typically involves computing the expected time of arrival (TOA) and subtracting it from the actual TOA to derive the time difference, subsequently used to calculate blade-tip displacement. However, calculating the expected TOA introduces computational errors. Moreover, obtaining blade-tip vibration displacement under variable rotational speeds necessitates pre-processing to remove trend terms caused by speed fluctuations. To address these challenges, this study introduces a paradigm shift by replacing the traditional displacement-based blade-tip timing (D-BTT) with velocity-based blade-tip timing (V-BTT). Specifically, this approach utilizes the angular information between adjacent sensors and the measured TOA to extract the blade-tip vibration velocity through a Taylor series expansion, thus eliminating the calculating of the expected TOA and improving the computational efficiency. Building upon this methodology, a compressed sensing model for undersampled vibration velocity is established and solved using the Block-OMP algorithm, enabling vibration parameter identification without relying on prior information. By providing a superior representation of mid-frequency components of blade-tip vibration, V-BTT facilitates precise identification of higher-order vibration parameters in multi-mode blade motion. Finally, the efficacy of the proposed method is confirmed through numerical simulations and experiments involving multi-mode blades. The experimental results reveal that the relative errors of the first two modal frequencies for the five blades identified using vibration velocity are all below 0.5%, affirming the method's accuracy. Compared to the traditional D-BTT method, the V-BTT approach reduces the need for trend removal, eliminates calculation errors associated with the expected TOA, and, most importantly, enhances the accuracy of multi-mode frequency identification.

Unsupervised CNN-based DIC method for 2D displacement measurement

Digital image correlation (DIC) is a widely used technique for non-contact measurement of deformation. However, traditional DIC methods face challenges in balancing calculation efficiency and the quantity of seed points. Deep learning approaches, particularly supervised learning methods, have shown promise in improving DIC efficiency. However, these methods require high-quality training data, which can be time-consuming to generate ground truth annotations. To address these challenges, we propose an unsupervised convolutional neural network (CNN) based DIC method for 2D displacement measurement. Our approach leverages an encoder-decoder architecture with multi-level feature extraction, a dual-path correlation block, and an attention block to extract informative features from speckle images with varying characteristics. We utilize a speckle image warp model to transform the deformed speckle image to the predicted reference speckle image based on the predicted 2D displacement map. The unsupervised training is achieved by comparing the predicted and original reference speckle images. To optimize the network's parameters, we employ a composite loss function that takes into account both the Mean Squared Error (MSE) and Pearson correlation coefficient. By using unsupervised convolutional neural network (CNN) based DIC method, we eliminate the need for extensive training data annotation, which is a time-consuming process in supervised learning DIC methods. We have conducted several experiments to demonstrate the validity and robustness of our proposed method. The results show a significant reduction in Mean Absolute Error (MAE) and Root Mean Squared Error (RMSE) compared to a method proposed by Zhao et al. This indicates that our unsupervised CNN-based DIC approach can achieve accuracy comparable to supervised CNN-based DIC methods. For implementation and evaluation, we provide PyTorch code and datasets, which will be released at the following URL :https://github.com/fead1/DICNet-corr-unsupervised-learning-.

Equivalent distortion imaging model-based calibration method for a projection moiré system using combined lenses

The projection moiré method is a noncontact full-field optical topography measurement technique, which requires a precise model and corresponding calibration method; however, the existing models and calibration methods are either incompatible with system distortion or involve complex experimental operations. This study presents an equivalent distortion imaging model coupled with a flexible calibration method that overcomes such shortcomings. The proposed model is constructed by considering the stacking effect of the distortions of the two lenses and the misalignment between the optical axes. The corresponding calibration method leverages the existing camera calibration method to avoid complex operations. The proposed model has high precision owing to its ability to describe the complex imaging of the system. Moreover, the calibration is flexible and requires far fewer operations than with the two-stage calibration method. Both simulations and experiments verify the effectiveness of the proposed method.

Substantiation for a new non-contact measurement technique in electrical resistance surveys

Background. Methods for electrical resistance surveys include those of electrical sounding and electrical profiling with various arrays. Measurements were originally carried out at direct current, although low-frequency alternating current was used later. In the case of direct current, the sole sources of the primary electric field in these methods comprise the charges of the A, B current electrodes, with the measured value being the electric field voltage E equal to the potential difference ΔUMN in the MN line with the measuring electrodes M, N. According to the measurement results, the apparent electrical resistivity is determined, which is convenient for interpreting the measurement results ρк. However, in some cases, e.g., when conducting measurements in permafrost areas, it can be difficult to ensure reliable grounding of the electrodes. Therefore, half a century ago, research was initiated to substantiate the possibility of contactless measurements in the method of electrical profiling with the alternating current I in the AB line. Until recently, the technique of non-contact measurements and interpretation of the results obtained has been based on approximate approaches, rather than on a strict solution of the forward problem of electrodynamics.Aim. Objective substantiation of the non-contact measurement technique based on the solution of a forward electrodynamics problem.Materials and methods. The data obtained by mathematical simulation were analyzed.Results. The results of calculations for a model corresponding to the possible conditions for contactless measurements in electrical exploration by the resistance method are presented. A case is considered when a generator line AB of a harmonically varying current I is located in the air, at a height h above a homogeneous conducting half-space with a specific electrical ρ2.Conclusion. In comparison with the method currently used for non-contact measurements, it seems more effective to determine the apparent electrical resistivity from the reactive component of the electric field voltage E in the measuring line MN that varies in phase with the current I in the generator line AB.

Development of Non-Contact Measurement Techniques for Concrete Elements Using Light Detection and Ranging

Development of Non-Contact Measurement Techniques for Concrete Elements Using Light Detection and Ranging

Aeroelastic response of a thin panel excited by a separated shock–boundary layer interaction

Wind-tunnel experiments were conducted to investigate the effects of a separated shock–boundary layer interaction on the aeroelastic behavior of a thin panel in turbulent flow. A suite of traditional sensors and non-contact measurement techniques, including stereo digital image correlation (DIC), were used to characterize the operating conditions and structural response. The impact of the temperature differential between the panel and frame as well as cavity pressure on the panel response were explored. For a majority of the operating conditions, measured time histories of the panel dynamics revealed small-amplitude, forced oscillations about a stationary deformation. These deformations were due to the pressure fluctuations from the turbulent boundary layer and separated shock/boundary layer interaction. However, for a cavity pressure of 83 kPa and temperature differentials of [15, 17.2] K, the response transitioned to bi-stable, cross-well oscillations that were captured for the first time using DIC. The dynamic response was centered about the panel midpoint, coincident with the location of the shock-induced separation on the panel. Using several indirect tests, the intermittent, cross-well oscillations were characterized as potentially chaotic. As the temperature differential continued to decrease, the snap-through motions subsided and the panel response resumed a stationary behavior, albeit at a different deformation shape.

Review of research progress and development trend of digital image correlation

PurposeThe purpose of this paper is to provide a comprehensive review of a non-contact full-field optical measurement technique known as digital image correlation (DIC).Design/methodology/approachThe approach of this review paper is to introduce the research pertaining to DIC. It comprehensively covers crucial facets including its principles, historical development, core challenges, current research status and practical applications. Additionally, it delves into unresolved issues and outlines future research objectives.FindingsThe findings of this review encompass essential aspects of DIC, including core issues like the subpixel registration algorithm, camera calibration, measurement of surface deformation in 3D complex structures and applications in ultra-high-temperature settings. Additionally, the review presents the prevailing strategies for addressing these challenges, the most recent advancements in DIC applications across quasi-static, dynamic, ultra-high-temperature, large-scale and micro-scale engineering domains, along with key directions for future research endeavors.Originality/valueThis review holds a substantial value as it furnishes a comprehensive and in-depth introduction to DIC, while also spotlighting its prospective applications.

Precise Phase Measurement for Fringe Reflection Technique through Optimized Camera Response

The Fringe Reflection is a robust and non-contact technique for optical measurement and specular surface characterization. The periodic alternation between dark and light cycles of the fringe patterns encodes the geometric information and provides a non-contact method of spatial measurement through phase extraction. Precisely expressing the positions of the points of the fringe pattern is a fundamental requirement for an accurate fringe reflection measurement. However, the nonlinear processes, both in generating the fringe pattern on a screen and capturing it using pixel values, cause inevitable errors in the phase measurement and eventually reduce the system’s precision. Aiming at reducing these nonlinear errors, we focus on constructing a new quantity from the pixel values of the photos of the fringe patterns that could linearly respond to the ideal fringe pattern. To this end, we hypothesize that the process of displaying the fringe pattern on a screen using a control function is similar to the process of capturing the pattern and converting the illuminating information into pixel values, which can be described using the camera’s response function. This similarity allows us to build a scaled energy quantity that could have a better linear relation with the control function. We optimize the extracted camera response function using an objective to increase the precision and reduce the quoted error. Experiments designed to determine the positions of points along the quartile lines verify the effectiveness of the proposed method in improving fringe reflection measurement precision.

A novel non-contact measurement strategy for large-size inflatable structures based on numerical predictions

The deformation characteristics of expansive inflatable structures, like airships, provide essential data for their operation in the stratosphere. However, due to rapid configuration changes in these structures with fluctuating air pressure, obtaining accurate morphological data quickly during testing can be challenging. This paper presents a precise method for predicting deformation in large-scale inflatable structures. A finite element method was employed to meticulously model the inflatable structure, underpinned by an inflatable ring skeleton support, and predict its potential maximum deformation points. Leveraging this predictive model as a guide, we proposed utilizing a laser scanning technique for non-contact measurements of the anticipated maximum deformation sites, aiming to discern the deformation patterns of the structure under varying air pressures. A good agreement between the non-contact measurement results and the numerical simulations affirms the efficiency and precision of the proposed methodology. This non-contact measurement strategy, grounded in finite element predictions, streamlines the measurement process, economizing on time and reducing complexity. Such an approach promises to be invaluable for expansive structures like airships. Furthermore, the insights derived from this non-contact method will be pivotal for anticipating the mechanical behaviors of full-scale stratospheric airships during their operations and for refining their design models.

Prospective clinical validation of a noncontact vital signs measurement smartphone application in emergency department

Abstract Background Remote photoplethysmography (rPPG) is a noncontact optical measurement technique that can be used to detect blood volume changes in the microvascular bed of tissue, using a RGB camera. We developed a smartphone application that leverages rPPG in order to monitor the heart rate and respiratory rate of patients using a 30s video selfie. Due to the widespread availability of cameras in smartphones, tablets and laptops, such an application allows to automate noncontact physiological monitoring and recording, improving the quality and accessibility of this essential clinical information. Purpose This study aimed to evaluate the accuracy of such remote measurements compared with regular contact based measurement methods (pulse oximeter) in real-life clinical settings. Methods We recruited 443 patients who consulted the emergency department in 4 French hospitals. After obtaining written informed consent, the patients filled in a questionnaire collecting demographic information including age, gender and the Fitzpatrick skin phototype. Each patient was asked to sit in front of the tablet, with the reference pulse oximeter installed on their finger. All measurements were 30s long. When starting the measurement, the medical staff started to count the patient's respiratory cycles. After 15s, the first COBOX result was available, and then updated every second. At the end of the 30s, the medical staff reported the number of respiratory cycles which were doubled to obtain a respiratory rate in cycles per minute (cpm). Results The mean age was 44 ± 18 years old, 45% was male. A total of 6311 pairs of values of heart rate taken by COBOX (HRCOBOX) and by the reference pulse oximeter (HRreference) and 453 pairs of values of respiratory rate taken by COBOX (RRCOBOX) and by the reference manual count (RRreference) were collected. The proportion of heart rates > 100bpm (beats per minute) was 4.4% and of respiratory rates > 20cpm was 22.9%. The number of insufficient quality measurements was 838 for heart rate (13%). The medical staff could not count reliably the respiratory rate in 20 sessions (4.4%). 30 sessions (6.9%) had a measurement of insufficient quality. For the heart rate, the Pearson correlation between COBOX and the reference was 97.2%. The root mean square error (RMSE) was 3.1bpm. For the respiratory rate, the Pearson correlation between COBOX and the reference was 78.3%. The RMSE was 2.9cpm. Conclusion Our work aimed to evaluate the accuracy of such remote measurements compared with existing contact point measurement methods in real-life clinical settings. Our results described a very good accuracy and correlation between the rPPG system and the gold standard, thus paving the way for more precise and more accessible remote patient monitoring. Such a technology has the potential to be transformative for telemedicine, especially in medical deserts where the access to both physicians and medical devices is lacking.Heart rate resultsRespiratory rate results

Micro-infrared thermometry for characterizing microscale heating devices

Infrared thermometry is a non-destructive and non-contact temperature measurement technique that allows for real-time, full-field imaging. However, the limited spatial resolution of infrared detectors and the scarcity of mid- and far-infrared lenses have hindered its use in microscale applications. To address this, we introduce an infrared microscope that employs a bolometer-type infrared detector and a reflective objective lens, enabling high-resolution temperature measurements of microscale heating devices. The reflective objective lens used in the microscope introduces intensity reduction due to magnification and undesirable background radiation caused by its cavity effect. To mitigate these issues, we utilized a custom-built micro-blackbody for calibrating intensity reduction and eliminating background radiation. The proposed micro-infrared thermometry setup was characterized by MDTD and MRTD measurements, as well as an MTF curve with a pixel resolution of 6.7μm/pixel and a spatial resolution of 12.5μm. To evaluate the effectiveness of the micro-infrared thermometry, we fabricated a suspended microscale heating device that can exhibit a clear temperature distribution. The temperature distribution obtained using both the differential conversion and Fast Fourier-Transform methods to eliminate background radiation agreed well, with a maximum difference of 3.2°C and 4.1°C in the range from room temperature to 77°C and 145°C, respectively. Furthermore, we compared our experimental results with COMSOL simulations and found them to be reasonably consistent, with a maximum difference of 6.1°C and 7.0°C in the range from room temperature to 77°C and 145°C, respectively.

Modeling the measurement precision of Fringe Projection Profilometry

Three-dimensional (3D) surface geometry provides elemental information in various sciences and precision engineering. Fringe Projection Profilometry (FPP) is one of the most powerful non-contact (thus non-destructive) and non-interferometric (thus less restrictive) 3D measurement techniques, featuring at its high precision. However, the measurement precision of FPP is currently evaluated experimentally, lacking a complete theoretical model for guidance. We propose the first complete FPP precision model chain including four stage models (camera intensity, fringe intensity, phase and 3D geometry) and two transfer models (from fringe intensity to phase and from phase to 3D geometry). The most significant contributions include the adoption of a non-Gaussian camera noise model, which, for the first time, establishes the connection between camera’s electronics parameters (known in advance from the camera manufacturer) and the phase precision, and the formulation of the phase to geometry transfer, which makes the precision of the measured geometry representable in an explicit and concise form. As a result, we not only establish the full precision model of the 3D geometry to characterize the performance of an FPP system that has already been set up, but also explore the expression of the highest possible precision limit to guide the error distribution of an FPP system that is yet to build. Our theoretical models make FPP a more designable technique to meet the challenges from various measurement demands concerning different object sizes from macro to micro and requiring different measurement precisions from a few millimeters to a few micrometers.

Experimental investigation on the nonlinear dynamic responses of the coupled floating submerged artificial seabed-mooring system in internal solitary waves

A new deepwater artificial seabed (DAS) production system is developed as a competitive offshore field development solution in deep and ultra-deep waters. This paper experimentally investigates the acting mechanism of ISWs on the nonlinear dynamic responses of the coupled floating submerged artificial seabed-mooring (ASM) system in a large-scale density-stratified flume. A compositive method for measuring and analyzing the interactions between the ASM system and the ISW is proposed. On this basis, the spatio-temporal features of the dynamic motion responses, the mooring tension variations and the flow field surrounding the ASM coupling system are characterized. The minute motion responses of the test model are captured by a charge-coupled device camera and subsequently processed using a developed image processing method. The temporal tension variations in the mooring lines are measured utilizing a non-contact optical measurement technique. The fluid-structure interaction characteristics between the flow field and the test model are revealed through the particle image velocimetry technique. In addition, the effects of crucial environmental factors on the nonlinear coupled dynamic responses of the ASM system are systematically explored in the experiments. The results indicate that the nonlinear dynamic responses of the ASM system in ISWs are characterized by the significant surge motion (maximum: 14.7% of the water depth), the coupled heave motion with the surge response, and the considerable mooring tension variation (maximum: 31.8% of the pre-tension), especially with the test model located within the density pycnocline.

Challenges in assessing the vibrations influence on people in buildings using non-contact measurements

This research article presents a~comparative analysis of vibration assessments in lecture halls to investigate their influence on people using contact (accelerometers) and non-contact (laser vibrometers) measurement techniques. The study aims to verify the accuracy and reliability of vibration analysis in relation to two approaches and determined physical parameters, i.e. acceleration amplitudes and vibration velocities. The intriguing fact was that none of the building users reported any perceived discomfort from vibrations, despite the determined parameters of the signal measured using a~laser vibrometer indicating exceedance of permissible vibration amplitudes in several frequency bands. The conducted comparative analysis leads to the conclusion that the location of the laser head tripod on the vibrating floor introduces significant vibration amplification, which in turn may lead to an incorrect assessment of the impact of vibrations on people in buildings. The studies described in the article were carried out in accordance with the procedure contained in the Polish national standard PN-B-02171. The obtained results and the resulting conclusions are an important contribution to a~better understanding of the advantages and limitations resulting from the use of non-contact measurements.

Assessment of out‐of‐plane structural defects using parallel laser line scanning system

AbstractA precise parallel laser line scanning system has been developed to assess the depth of out‐of‐plane structural defects on concrete surfaces. This system comprises a digital camera, dual line laser diodes, and positioning rigid arms that create a triangulation‐based setup. Laser lines are distorted when projected onto an out‐of‐plane defect. A new image processing algorithm has been devised to extract depth information from the distorted laser strips. Parallel laser lines are used to ensure that imaged laser strips do not intersect, thereby simplifying the depth assessment of defects at different distances and enabling the generation of defect profiles from a single image. The system has been validated through laboratory and field tests, demonstrating its effectiveness and accuracy. Compared to other noncontact measurement techniques, this system stands out due to its simplicity, cost‐effectiveness, efficiency, and superior accuracy for long‐range measurements, rendering it suitable for on‐site scanning of textureless uneven engineering objects.

Simultaneous measurement of refractive index and dispersion using optical coherence tomography for restoration of transparent works of art

We describe a novel in situ non-invasive and non-contact technique for simultaneous measurement of refractive index and dispersion of transparent solid materials using optical coherence tomography (OCT). The technique requires multi-angle OCT imaging. It gives typical precision of 0.02 for the refractive index and 0.03 mum^{-1} for dispersion. The method can be applied to the restoration of modern sculptures made from plastics where it is important to have precise measurement of the refractive index of an object using a non-contact method, so that a resin-based adhesive with a matching refractive index could be found to restore a scratched or broken transparent works of art.

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.

Video Motion Magnification and Subpixel Edge Detection-Based Full-Field Dynamic Displacement Measurement

Noncontact measurement techniques in structural dynamics field have progressed significantly in the past few decades. Vision-based measurement techniques are unique in that they have the ability to achieve full-field measurement and possess the typical advantages associated with noncontact measurement techniques. Recently, vision-based techniques have also been applied to streaming of videos for structural dynamic displacement measurement. The most recent trends in vision-based measurements include target tracing, digital image correlation, and target-less approaches. There are, however, some shortcomings of the vision-based techniques such as susceptibilities to image noise, prevailing light conditions, and limit in measurement resolution. To reduce these shortcomings, a method known as video motion magnification (MM) can be used to amplify small structural motions. Using the phase-based motion magnification (PBMM) and subpixel edge detection methods, the full-field dynamic displacements of the structure can be obtained. The deep convolutional long short-term memory (ConvLSTM) network is applied to aid in the selection of the frequency band for magnification in the PBMM algorithm. To achieve higher measurement accuracy, the displacement results with and without MM are combined with the finite impulse response (FIR) filter which can reduce the error caused by the PBMM procedure. In the tests, plastic optical fiber (POF) displacement sensors are introduced and used as reference measurements to compare the dynamic displacement results from the proposed vision-based method. Compared with the measured displacements with POF sensors, the proposed method offers high level of accuracy for full-field displacement measurement.

Non-contact measurement techniques to study the microsized particle adhesion phenomenon

Airborne microsized particles are generated by artificial activity and natural sources. Due to their characteristics, these airborne particles are highly volatile and can cover thousands of kilometers according to weather conditions (in particular, wind intensity and humidity). As a result, these microsized particles could contaminate engineering systems determining their operation and performance modification. Phenomena such as photovoltaic panel soiling or gas turbine fouling are two of the most detrimental effects generated by the interaction between airborne particles and the relevant surfaces of the system. The present work proposes a set of non-contact measurement methodologies to study the adhesion phenomenon. The non-contact techniques are devoted to studying the deposited layer without altering the deposits. Image and video analysis have been used to show how the deposit can be studied in detail. Stationary and rotating facilities have been employed to show the applicability and the related constraints to the non-contact measurement techniques.

Increasing the Frequency Resolution when Measuring Vibrations of Rotating Bodies with Fixed Beam Laser Vibrometry

Laser Doppler vibrometry is actively used in experimental studies because of its noncontact measurement technique. When using a stationary laser to measure the vibrations of rotating bodies and Fourier transform to process the results of such measurements, a problem arises, associated with a decrease in the frequency resolution of the spectra with increasing rotation rate of the body. As a result, at sufficiently high rotation rates, closely spaced discrete components may cease to be resolved. This paper proposes a method for solving such a problem using the least squares method. The operability of this processing method has been demonstrated on experimental data.