Non-contact Displacement Measurement Research Articles

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.

Improving displacement measurement accuracy by compensating for camera motion and thermal effect on camera sensor

Displacement measurement is a major concept in the daily maintenance and failure assessment of engineering structures. Vision-based systems offer a cost-effective, simple alternative for non-contact displacement measurement. However, its measurement is challenging because of the errors induced by camera instability (thermal effects on camera sensor and camera motion). Researchers have tried to compensate for the errors induced by camera instability in displacement calculation methods by placing fixed objects in the background. However, remotely-located objects in the background cannot fully allow for camera rotations. Supplementary sensors can be employed to measure the camera motion in six degrees of freedom (6-DOF), although this method is unable to compensate for thermal effects on camera sensor. This paper presents a dual-view system that can compensate for the camera instability by using a camera, a beam splitter, a convex lens, and a photomask. The beam splitter and convex lens enable the camera to capture dual-view data and measure the instability of the system by using the photomask. In laboratory tests, the Root Mean Squared Error (RMSE) of the displacements measured by the proposed system did not exceed 0.05 mm under the conditions of drastic temperature variations and strong winds. Outdoor tests showed that the reflection of sunlight on the objects will seriously affect the practical performance of the proposed system, and a robust localization method for the photomask is necessary. The measurement scheme design for the track structure vibration of high-speed railways proved the acceptable engineering practicability and installation flexibility of the dual-view system.

Model analysis of bucket wheel excavator SchRs 630 strength

The possibilities of testing real constructions are often limited, especially in case of large constructions. The idea of model testing is to create a sub-scaled model in order to anticipate the behavior of the real construction, regarding the behavior of the model, with sufficient accuracy. The aim of this paper is to demonstrate that model testing of bucket wheel excavator structure, from the aspect of the strength of the constructions, is possible. In order to prove this assumption, firstly, numerical calculations of a real construction and its sub-scaled model were performed using the Finite Element Method. Sub-scaled structure is 10 times scaled real structure (in size and plates thickness). Within the static calculation, the analysis of the stress mapping from sub-scaled model to real structure was carried out, by partial groups of construction. The coefficients connecting stresses of sub-scaled model and stresses of real construction were formulated. Dynamic calculations of sub-scaled model and real structure were also performed. Then, a physical model was produced and subjected to experimental testing. Experimental testing was performed in order to verify the numerical models. A parallel testing of the model with strain gauges and a system for non-contact measurement of displacement and deformation was performed. High matching results were obtained of two experimental methods on one side, and numerical calculation on the other side. The physical model and the successful experiments carried out on it confirmed that these structures can be subjected to model testing, without important constraints and with sufficient accuracy.

Non-contact structural displacement measurement using Unmanned Aerial Vehicles and video-based systems

This article describes an innovative methodology for estimating in-plane horizontal displacements of Civil Engineering structures based on video systems integrated on Unmanned Aerial Vehicles (UAVs). As the structure and the UAV are both in motion, estimating absolute structural displacements involves, first, assessing the relative displacements of the UAV-structure based on target tracking, and second, subtracting movement of the UAV based on the data derived from an embedded Inertial Measuring Unit (IMU). Computer-vision processing tools based on heuristic features were developed for target tracking, while an efficient numerical integration strategy was implemented for processing IMU data. A successful integration process requires a baseline correction of the records through the application of high-pass filters and a rigorous control of distortion errors. The validation of the proposed methodology was based on exploratory dynamic tests performed in the laboratory and in the field.The laboratory test involved measuring the displacements of a moving target, positioned over a seismic table using a Linear Variable Differential Transformer (LVDT), a DC accelerometer and the UAV video system in a stationary position. An excellent agreement was achieved for the three estimated displacements, both in the time and frequency domains. In particular, the comparison between the video and the LVDT displacement records showed a peak value error of 0.096 mm and a Root Mean Square (RMS) error of 3.1%.The field test consisted of measuring the displacements of a target fixed to a massive Reinforced Concrete (RC) wall based on the UAV video system in motion. Under these circumstances, the target does not experience any movement, and therefore, the virtual displacement of the target estimated by the video is due exclusively to the self-movements of the UAV. The comparison between the video and the IMU displacement records shows a good agreement, with a peak value error of 1.47 mm (15.5% relative error) and a RMS error of 9.3%.The results of this study are a step forward in estimating the absolute displacements of a vibrating structure based on camera-based measurement from UAVs, specifically with the support of IMU systems for the elimination of measurement inaccuracies as a result of the UAV motion.

Fracture toughness of ZrO2–SiC/MoSi2 composite ceramics prepared by powder metallurgy

ZrO2–SiC/MoSi2 composite ceramics were prepared by powder metallurgy. The room temperature and high temperature fracture toughness of 10 vol%ZrO2+MoSi2, 10 vol%ZrO2+10 vol%SiC + MoSi2, 20 vol%ZrO2+10 vol%SiC + MoSi2, 10 vol%ZrO2+20 vol%SiC + MoSi2 composite ceramics were studied. The room temperature fracture toughness was calculated by Vickers indentation crack. High temperature fracture toughness is measured by wavefront interference principle of moire interferometry and non-contact measurement of in-plane displacement. The synergistic effects mechanism of nano-ZrO2 and SiC particles on the fracture toughness of MoSi2 nanocomposite ceramics was discussed.

Application of integrated binocular stereo vision measurement and wireless sensor system in athlete displacement test

Based on binocular stereo vision measurement and wireless sensing theory, this paper studies the measurement and analysis methods of athletes' movement displacement parameters. The main purpose is to explore the relationship between sports biomechanical parameters. The analysis reveals the laws of athletes 'movements, and provides theoretical guidance on athletes' attitude recognition and physical training. Based on the principle of binocular stereo vision, a non-contact displacement measurement system was developed. The displacement measurement system uses template matching to extract the image coordinates of the measurement points and restore the spatial coordinates of the measurement points through European 3D reconstruction, which improves the practicality of the measurement system. In the cantilever beam vibration test, the traditional displacement sensor and binocular image displacement measurement system are used to simultaneously measure the cantilever beam displacement. The experimental results show that the integrated displacement measurement system in this paper has higher accuracy and better stability.

Development of a Compound Speckle Interferometer for Precision Three-Degree-of-Freedom Displacement Measurement.

In this study, a compound speckle interferometer for measuring three-degree-of-freedom (3-DOF) displacement is proposed. The system, which combines heterodyne interferometry, speckle interferometry and beam splitting techniques, can perform precision 3-DOF displacement measurements, while still having the advantages of high resolution and a relatively simple configuration. The incorporation of speckle interferometry allows for non-contact displacement measurements by detecting the phase of the speckle interference pattern formed from the convergence of laser beams on the measured rough surface. Experiments were conducted to verify the measurement capabilities of the system, and the results show that the proposed system has excellent measurement capabilities suitable for future real-world applications.

Computer vision-based displacement measurement with m-sequence target

The development of image sensors enables the application of vision-based techniques to the non-contact displacement measurement of large-scale structures. The features of the physical targets are critical to the accuracy, stability and anti-interference of the displacement measurement results. In this study, a novel m-sequence target and the associated circular correlation processing technique are developed for real-time displacement measurement. The properties of the m-sequence as a pseudo-random sequence are introduced. The vision-based displacement calculation method is then derived from the correlation property of the m-sequence. The algorithms and measurement systems are integrated in the LabVIEW environment. To verify the anti-interference performance of the developed system, static and dynamic experimental tests are carried out with various forms of interference, such as partial occlusion, uneven illumination, out of focus and smoke effect. Experimental results indicate that the developed system cannot only accurately measure structural displacement, but also has outstanding antiinterference performance, even if 30% of the target is masked.

A novel high-frequency fatigue testing methodology for small thin-walled structures in the HCF/VHCF regime

A novel high-frequency testing methodology for small thin-walled component-like structures is presented that enables fatigue tests up to 1e9 cycles within 7–8 days. A shaker excites the designed thin-walled (min.0.1 mm) specimen near its resonant frequency and a non-contact relative displacement measurement allows to set a defined notch stress. Calculated and experimentally determined resonant frequencies deviate max.by −1.9%. Furthermore, vibration and temperature measurements prove the stability of the technique, no self heating of the specimen is detected. Validation tests with an X5CrNiCuNb16-4 steel show a typical S-N behaviour for this material, whereby cracks always appear in the intended notch.

Calculating the longitudinal and vertical displacements of a moving object by digital image processing methods

In the article, algorithms for calculating the longitudinal and vertical displacement of a moving object relative to a stationary base by a non-contact method using a laser-optical measuring de-vice are described and compared. The problem of non-contact measurement of longitudinal and vertical displacement belongs to the class of problems solved by methods of computer vision - image processing for measuring physical quantities. Two methods of calculating the value of the longitudinal and vertical displacement are compared. The first is based on the distance to the object, the size of the image sensor using the thin lens equation. The second is based on the method of calculus of distance to the object and the angle of view of the video recorder using the tangent formula. The measurements were carried out for the range of 0.5 - 500 m. The results of the work can be used to create high-precision positioning systems and high-precision vehicle guidance systems for various types of transport (sea, river, road, air and rail). Such systems solve the problem of stabilizing the position of moving objects in various transport systems, as well as position control.

Piezoelectric mirror shifter transfer function measurement, modelling, and analysis using feedback based synthetic-heterodyne Michelson interferometry

Laser vibrometry has many applications in non-contact dynamic displacement and vibration measurement. A test beam reflected from a target and a reference beam are combined and detected by a photodiode; the photodetected signal is then processed to determine the target displacement and vibration. This paper describes the use of a 9 kHz measurement bandwidth system, consisting of a Michelson interferometer and self-correcting feedback synthetic-heterodyne signal processing technique, to measure the displacement impulse response of a commercial piezoelectric mirror shifter (PMS), consisting of a mirror mounted on a Piezoelectric transducer and a connecting 50 Ω electrical coaxial cable. The actual non-ideal applied impulse and measured impulse response data were used in conjunction with the instrument variable method to determine a Laplace domain linear transfer function approximation to the actual PMS transfer function. The best transfer function fitting, having a 84% normalized root mean square goodness of fit, was obtained using a 5-th order transfer function having two complex conjugate pole pairs, with associated natural frequencies of 6.29 and 6.79 kHz, and a single real pole. The transfer function zeros consisted of a single complex conjugate zero pair, having an antiresonance frequency of 6.38 kHz and a single real zero. Knowing the analytic transfer function of PMS based nanopositioners is useful for example in the design of closed-loop phase-locked interferometers for wideband sensing.

A smartphone camera and built-in gyroscope based application for non-contact yet accurate off-axis structural displacement measurements

Image-based optical methods have been widely used for noncontact structural displacement measurements, due to their prominent advantages over conventional contact sensors. However, existing optical methods usually require complicated and expensive imaging systems, and have difficulties to accurately measure in-plane displacements when the optical axis is not perpendicular to surface of the object (i.e., off-axis measurement). In this work, we develop a low-cost and portable smartphone-based optical method for accurately measuring off-axis structural displacements. The theoretical equations of the in-plane physical displacements on the object surface are derived based on the smartphone gyroscope data that can be used to determine the rotation matrix between the defined world coordinate system and the camera coordinate system. Simple calibration tests are performed to validate the accuracy of the smartphone gyroscope in detecting the rotation angles. The effectiveness and accuracy of this method in off-axis structural displacement measurement are then verified by two in-plane translation tests in laboratory condition. Finally, the noise from the smartphone camera is evaluated, and the effectiveness of the proposed method in continuous displacement measurement is further confirmed the vibration measurement of a hanging light under random wind load.

Sensors for Road Tests and Test Rigs

Micro-Epsilon discusses some of the more challenging vehicle test applications for non-contact displacement measurement and draw-wire sensors

Swaying displacement measurement for structural monitoring using computer vision and an unmanned aerial vehicle

Data acquisition is the challenging and crucial step for any structural health monitoring (SHM) scheme, especially on numerous measurement locations that are typically at very high elevations or largely inaccessible areas, which are often linked to time-consuming, costly, and to some extent, dangerous sensor implementation and cable wiring. Noncontact vision-based measurement techniques have been recognized recently as a primarily feasible approach, although it is still characterized by some limitations. To address these constraints, the proposed study introduced an enhanced noncontact displacement measurement method that employed an unmanned aerial vehicle (UAV) and computer vision algorithms. Since UAV can carry cameras to approach any difficult-to-reach regions, the proposed system can overcome several bottlenecks of the state-of-the-art vision-based methods with regard to finding a stationary place for the camcorder and for mitigating the inaccuracy induced by the long distance between the camcorder and the measurement location. Guided by the schematic framework for the system, a camera was mounted on the UAV for filming of the measurement point, and then displacements on that point were determined by a key-point vision-based measurement method. Moreover, translations generated by the UAV were obtained by means of reference objects on the background. Additionally, an autonomous scheme based on Canny edge detection and Hough transform were introduced for calculation of scale factors between the pixel and engineering unit for every image frame to address the issue of very fluctuant distances from the UAV to the measurement location. Subsequently, the actual displacements of the measurement location were measured following the elimination of the UAV motions from the displacement data. The proposed system was verified on an experiment with a small-sized steel tower where the outcomes provided an initial confirmation of the approach’s promising potential.

Non-contact and high-precision displacement measurement based on tunnel magnetoresistance

High-precision displacement measurement is essential to various industrial fields, but the drawbacks of existing sensors like bulkiness, low compatibility and high cost limit their applications. In this study, a non-contact and high-precision displacement measurement method based on tunnel magnetoresistance is introduced to achieve high accuracy and small size. The displacement of a permanent magnet changes the spatial distribution of the magnetic field and thus the output of the full-bridge tunnel magnetosresistance (TMR)-based component varies. Based on the theoretical analysis, the highest sensitivity of the measurement system is obtained and the corresponding layout of the permanent magnet and TMR component to ensure the system achieves such sensitivity is determined. The results are validated by experiments, and the highest sensitivity is 17.15 mV m−1. The system shows stable and high-quality frequency response below 300 Hz. The resolution reaches to 33 nm and the system is of high resolution compared with the eddy current sensor. The time drift is determined to be 0.121 m h−1, which is 30% lower than the existing laser sensor. All the features of the measurement system imply that it is of high accuracy and robustness. The compact structure and low cost make it a promising alternative to existing displacement sensors and demonstrates its potential for integration with the actuator.

Instability mechanism of shallow tunnel in soft rock subjected to surcharge loads

The tunnel instability accidents caused by surcharge loading could even lead to ground settlement or collapse, damage to the existing municipal pipelines or inclination of the surgical buildings. Series of large-scale physical model test was conducted in this paper to reveal the failure mechanism of tunnel in soft rock subjected to surcharge loading. The non-contact full-field displacement measurement (i.e. Digital Image Correlation, DIC) was employed to obtain a detailed deformation of surrounding rock after excavation. A coupled numerical method combing the continuum (Finite Difference Method, FDM) and dis-continuum (Discrete Element Method, DEM) analysis was used to deal with the failure mechanism of tunnel excavation in rock mass. Good agreements between physical model test and numerical analysis approve the accuracy of the proposed FDM-DEM numerical model. It is found that the collapse mainly occurred in tunnel roof rather than in tunnel sidewall. The collapse zone shows a step-type increase with the increase of surcharge loads. The stress loosening zone (SLZ) in tunnel roof is larger than that in tunnel sidewall. The thickness of SLZ in tunnel roof can be used to design the length of the formed pressure anchors. The proposed large-scale model test with DIC method is applicable to the tunnel engineering study. Moreover, the results in this paper give some insights to secure excavation works and optimize supporting structures.

Experiment of Structural Geometric Morphology Monitoring for Bridges Using Holographic Visual Sensor

To further improve the precision and efficiency of structural health monitoring technology and the theory of large-scale structures, full-field non-contact structural geometry morphology monitoring is expected to be a breakthrough technology in structural safety state monitoring and digital twins, owing to its economic, credible, high frequency, and holographic advantages. This study validates a proposed holographic visual sensor and algorithms in a computer-vision-based full-field non-contact displacement and vibration measurement. Using an automatic camera patrol experimental device, original segmental dynamic and static video monitoring data of a model bridge under various damage/activities were collected. According to the temporal and spatial characteristics of the series data, the holographic geometric morphology tracking algorithm was introduced. Additionally, the feature points set of the structural holography geometry and the holography feature contours were established. Experimental results show that the holographic visual sensor and the proposed algorithms can extract an accurate holographic full-field displacement signal, and factually and sensitively accomplish vibration measurement, while accurately reflecting the real change in structural properties under various damage/action conditions. The proposed method can serve as a foundation for further research on digital twins for large-scale structures, structural condition assessment, and intelligent damage identification.