Non-contact Displacement Measurement Research Articles

Exploration of temperature effect on videogrammetric technique for displacement monitoring

There has been a sustained interest towards the non-contact structural displacement measurement by means of videogrammetric technique. On the way forward, one of the major concerns is the spurious image drift induced by temperature variation. This study therefore carries out an investigation into the temperature effect of videogrammetric technique, focusing on the exploration of the mechanism behind the temperature effect and the elimination of the temperature-caused measurement error. 2D videogrammetric measurement tests under monotonic or cyclic temperature variation are first performed. Features of measurement error and the casual relationship between temperature variation and measurement error are then studied. The variation of the temperature of digital camera is identified as the main cause of measurement error. An excellent linear relationship between them is revealed. After that, camera parameters are extracted from the mapping between world coordinates and pixels coordinates of the calibration targets. The coordinates of principle point and focal lengths show variations well correlated with temperature variation. The measurement error is thought to be an outcome mainly attributed to the variation of the coordinates of principle point. An approach for eliminating temperature-caused measurement error is finally proposed. Correlation models between camera parameters and temperature are formulated. Thereby, camera parameters under different temperature conditions can be predicted and the camera projective matrix can be updated accordingly. By reconstructing the world coordinates with the updated camera projective matrix, the temperature-caused measurement error is eliminated. A satisfactory performance has been achieved by the proposed approach in eliminating the temperature-caused measurement error.

Application and Analyzation of the Vision‐Based Structure Model Displacement Measuring Method in Cassette Structure Shaking Table Experiment

In the shaking table test of large cassette structure, story drift is an essential set of experimental data. The traditional method of displacement measurement is limited to problems such as necessary full contact with the structure model for installation of sensors, large work of installation, and easily interfered by environment. The noncontact displacement measurement method, such as optical measuring technology, can solve the above problems and serve as an effective supplementary method for traditional displacement measuring in the shaking table test. This paper proposed a vison‐based displacement measuring method. Predesigned artificial targets which act as sensors are installed on each floor of the cassette structure model. A high‐speed industrial camera is used to acquire the series of the images of the artificial targets on the structure model during the shaking table test. A Python‐OpenCV‐based structural calculation program combining computer vision and machine vision is developed to extract and calculate the displacement of the artificial targets from the series of the images acquired. The proposed method is applied in a shaking table test of a reduced‐scale fifteen‐floor reinforced concrete cassette structure model, in which the laser displacement meter and the seismic geophone are also applied as a comparison. The experimental results acquired by the proposed method are compared with the results acquired by the laser displacement meter and the seismic geophone. The average error of the story drift obtained by the proposed vision‐based measurement method is within 5% and is in good agreement with the laser displacement meter and the seismic geophone, which confirms the effectiveness of the proposed method.

Design of a Tribotester Based on Non-Contact Displacement Measurements.

A tribotester with an integrated load sensor based on a strain gauge is typically used to measure the friction coefficient generated by the contact-related sliding motion of two objects. Since the friction coefficient is obtained by dividing the measured friction force by the applied normal force, the normal and friction forces must be measured for accurate analysis. In this study, a tribotester was used to measure the displacement of a cantilever tip using the fiberoptic sensor in a non-contact method. The friction coefficient measurement using the fiberoptic sensor was proven to be valid by calibrating the tip displacement due to normal/friction forces after designing a basic structural cantilever tip based on experiments and simulation analyses. The results obtained by using the fiberoptic sensor-cantilever tip-based tribotester were compared with those obtained using commercial and/or custom-built tribotesters under the same conditions. By designing various shapes of cantilever tips and using simulation analysis, the calibrations of the normal/friction forces and tip displacement could be verified and the coupling effect was evaluated. The performance and reliability of the fiberoptic sensor-cantilever tip-based tribotester, which can be used to determine the normal/friction forces by non-contact displacement measurements without a strain gauge, were verified.

Strength and toughness of hybrid steel and glass fiber-reinforced sulfur polymer composites

In this study, we investigated the effects of microfibers on the compressive and flexural behaviors of modified sulfur composites. Dicyclopentadiene-modified sulfur was used as the binder in the sulfur composites. Fifteen mix cases were tested by varying the volumetric ratios of steel and electric chemical resistant glass fibers. Fly ash (35% by volume) was included to not only increase strength and workability, but also ensure fiber dispersibility in the matrix. A non-contact displacement measurement technique, digital image correlation, was used in flexure tests to monitor the development of high strain zones and microcracks. The effect of fiber dosage on the porosity of sulfur composites was analyzed by quantifying the pore volume and size distribution through mercury intrusion porosimetry. The test results confirmed that the hybrid use of steel and glass fibers with a volume ratio of up to 4% was effective in improving the flexural stress-deflection response. The post-peak toughness ratio in flexure increased as the volume of microfibers increased. In addition, the total volume of hybrid microfibers generally had a positive correlation with the flexural strength. In contrast, the compressive strength of hybrid fiber-reinforced sulfur composites was more dependent on the portion of steel fibers than glass fibers.

A method for the non-contact measurement of two-dimensional displacement of chest surface by breathing and heartbeat using an airborne ultrasound

Vital-signs measurement systems, which have sensors that are brought into contact with or close to the body, are used for many applications. However, these systems are not suitable in some cases, namely when continuous monitoring is required or a subject cannot wear sensors. We have studied non-contact measurement of respiration and heart rate using an airborne ultrasound. In the proposed method, an M-sequence modulated ultrasound is transmitted to the chest of the subject, then the received signal is correlated with the transmitted M-sequence to detect the reflected echo with high signal-to-noise ratio. The time of flight (TOF) of the reflected echo indicates the distance to the chest surface. Therefore, the chest displacement due to breathing and heartbeat can be measured from the variation of TOFs. In this paper, the proposed measurement is demonstrated for subjects in a supine position without clothes. In the experiment, two cross-correlation functions were obtained by using one loudspeaker and two microphones to form the airborne B-mode image. Two-dimensional displacement of the chest surface due to breathing and heartbeat could be measured from the variation of airborne B-mode images.

Vibration Measurement of an Unbalanced Metallic Shaft Using Electrostatic Sensors

Vibration measurement of a rotary shaft is essential for the diagnosis and prognosis of industrial rotating machinery. However, the imbalance of a shaft, as quantified through vibration displacement, is the most common cause of machine vibration. The objective of this paper is to develop a novel technique through electrostatic sensing for the online, continuous and noncontact displacement measurement of a rotary shaft due to imbalance faults. A mathematical model is established to extract useful information about the shaft displacement vibration from the simulated signal in the frequency domain. Experimental tests were conducted on a purpose-built test rig to measure the displacement vibration of the shaft. An eccentric shaft was tested with the output signal from the electrostatic sensor analyzed. The effectiveness of the proposed method is verified through computer simulation and experimental tests. Results obtained indicate that the measurement system yields a relative error of within ±0.6% in the displacement measurement.

Failure Propagation of Pillars and Roof in a Room and Pillar Mine Induced by Longwall Mining in the Lower Seam

Underground mining using the room and pillar method leaves pillars to support the overlying strata. When longwall mining is used to extract a lower coal seam beneath the room and pillar mine, the bending of the interburden strata can induce pillar failure and roof caving. In this study, two physical model experiments with different pillar width/height ratios were performed, and a non-contact displacement measurement technique was used to record and analyse the deformation and failure processes. The failure mechanism of pillars and the roof in a room and pillar mine induced by longwall mining in a lower coal seam was investigated. The results show that, during the longwall retreat, the immediate roof of the longwall panel collapses and the main roof bends resulting in load redistribution among a series of overlying pillars. Eventually pillar failure and unsteady strata movement are induced. Physical model tests indicate the entire deformation and failure process can be divided into two stages, i.e. steady deformation and failure propagation. In addition, it is observed that different width/height ratios of pillars lead to different failure patterns in the second stage. For model #1 with higher pillar width/height ratios, load redistribution is induced by the bending of the main roof of the longwall mine, but most panel pillars remain stable. The unsteady movement of the main roof of the longwall causes an upward failure propagation in the overburden strata. For model #2 with lower pillar width/height ratios, the failure propagates downward and it is triggered by the progressive collapse of the panel pillars during the load redistribution process.

Output characteristics of two-circle coaxial optical fiber bundle with regard to three-dimensional tip clearance.

A two-circle coaxial optical fiber bundle is a type of optical sensor which has been widely used in non-contact radial displacement measurement applications. This paper has focused on output characteristics of a two-circle coaxial optical fiber bundle used as a measurement unit in a probe designed to measure a three-dimensional tip clearance. First, a model that could calculate intensity point-by-point on each receiving fiber with a simplified algorithm of overlap area was established, an aperture angle compensation algorithm was then proposed to improve the performance of the model. In order to test the reliability of the model, an experiment was done with different three-dimensional displacement of the reflector. Comparison between experimental and simulation results indicates the model built in this article could better describe how three-dimensional displacement of reflector affects the output of the bundle than previous studies.

Non-Contact Measurement of the Surface Displacement of a Slope Based on a Smart Binocular Vision System.

The paper presents an intelligent real-time slope surface deformation monitoring system based on binocular stereo-vision. To adapt the system to field slope monitoring, a design scheme of concentric marking point is proposed. Techniques including Zernike moment edge extraction, the least squares method, and k-means clustering are used to design a sub-pixel precision localization method for marker images. This study is mostly focused on the tracking accuracy of objects in multi-frame images obtained from a binocular camera. For this purpose, the Upsampled Cross Correlation (UCC) sub-pixel template matching technique is employed to improve the spatial-temporal contextual (STC) target-tracking algorithm. As a result, the tracking accuracy is improved to the sub-pixel level while keeping the STC tracking algorithm at high speed. The performance of the proposed vision monitoring system has been well verified through laboratory tests.

Design of a high-precision and non-contact dynamic angular displacement measurement with dual-Laser Doppler Vibrometers

A wide dynamic range, high precision, non-contact and large bandwidth angular displacement measurement (ADM) is greatly necessary for the applications such as industrial control and military equipment. This paper proposes a simple and effective ADM method based on Doppler Effect, heterodyne detection and diffuse reflection, which can fulfill these requirements simultaneously. Two beams of parallel light generated by a pair of laser Doppler vibrometers are incident upon the surface of rotational target, then data processing unit acquires the velocity of dual laser incident points on the moving target, and resolves the rotational angular displacement and translation displacement of target through the relationship between dual laser beams dynamically. Several major measurement errors that may affect the ADM accuracy are analyzed. A high precision rotary table is used as an angular displacement standard to verify the measurement range and accuracy, the verification experiment shows that the measurement range is not less than ±10° and the measurement accuracy is 0.0362° based on the method. After using a polynomial error compensation, the measurement accuracy can be promoted to 0.0088°, and this compensation method can be applied to real time measurement.

A Modal Expansion Method for Displacement and Strain Field Reconstruction of a Thin-Wall Component During Machining

Machining complex thin-wall components (such as compressor disks and casings in aircraft engines) has been a challenging task, because workpiece deformations and vibrations not only compromise the surface integrity, but also induce residual stresses in the final products. This paper offers a physics-based method that accounts for the damping effects and external loads for reconstructing the dynamic displacement and strain fields of a thin-wall workpiece in real time with noncontact displacement measurements during machining. Given that part dynamic behaviors can be characterized by superposition of mode shapes, the time-varying displacement and strain fields are reconstructed with modal coefficients that are updated in real time using in situ measurements. The reconstruction method has been numerically verified with finite-element analyses with the sensor locations optimized using a genetic algorithm; both static and dynamic field reconstructions are analyzed. Tradeoffs between the number of sensors and the reconstruction efficiency in terms of computation time and error are discussed. The method has been evaluated experimentally on a lathe machine testbed, where the dynamics of the distributed physical fields have been successfully captured and analyzed, demonstrating its practicality as a real-time tool for continuously monitoring the displacement and strain distributions across a disk workpiece during machining.

Sensing dynamic displacements in masonry rail bridges using 2D digital image correlation

Dynamic displacement measurements provide useful information for the assessment of masonry rail bridges, which constitute a significant part of the bridge stock in the United Kingdom and Europe. Commercial 2D digital image correlation (DIC) techniques are well suited for this purpose. These systems provide precise noncontact displacement measurements simultaneously at many locations of the bridge with an easily configured camera set-up. However, various sources of errors can affect the resolution, repeatability, and accuracy of DIC field measurements. Typically, these errors are application specific and are not automatically corrected by commercial software. To address this limitation, this paper presents a survey of relevant DIC errors and discusses methods to minimise the influence of these errors during equipment set-up and data processing. A case study application of DIC for multipoint displacement measurement of a masonry viaduct in Leeds is then described, where potential errors due to lighting changes, image texture, and camera movements are minimised with an appropriate set-up. Pixel-metric scaling errors are kept to a minimum with the use of a calibration method, which utilises vanishing points in the image. However, comparisons of DIC relative displacement measurements to complementary strain measurements from the bridge demonstrate that other errors may have significant influence on the DIC measurement accuracy. Therefore, the influence of measurement errors due to lens radial distortion and out-of-plane movements is quantified theoretically with pinhole camera and division distortion models. A method to correct for errors due to potential out-of-plane movements is then proposed.

Accurate and Robust Displacement Measurement for FMCW Radar Vibration Monitoring

Non-contact displacement measurement based on the phase-based microwave interferometry technology has attracted growing interest in recent years. In this paper, aiming to exploit the frequency-modulated continuous wave (FMCW) radar-based vibration monitoring technique, the principle and detailed procedure for extracting the displacement time series of vibration movement are illustrated. The approximate maximum likelihood approach is proposed to extract the phase history of beat signals across multiple sweep periods. Moreover, the measurement accuracy of vibration displacement extraction and several practical issues of this technique are evaluated and concerned. Both simulated and experimental validations are provided to show the effectiveness and robustness of the proposed method and approach.

Examples of modern methods of measuring deformations

The work presents examples of possibilities created by multi-unit 3D correlation system. Experience is the non-contact measurement of displacement and strains of slender compression bars or tensile samples. Experimental research show that as a result of axial compression of slender elements with the length substantially exceeding the cross-sectional dimensions, element destruction occurs at loads less than the respective compressive strength, resulting in deformation of the object. We present the results of numerical analysis, compared with exercise of carrying on a test bench.

Radial Throw at the Cutting Edges of Micro-Tools When Using Ultra-High-Speed Micromachining Spindles

In this paper, we present an approach to determine the trajectory of the cutting edges of a micro-tool in the presence of radial throw when using ultra-high-speed (UHS) micromachining spindles. When micro tools are rotated, inaccuracies and dynamic response of the tool-collet-spindle assembly cause the microtool axis to displace radially from the average axis of rotation. This displacement is referred to as the radial throw of the axis of rotation. Unlike runout, which defines the total radial displacement of a surface within one cycle of rotation, radial throw depends on the angle of rotation (and thus, the time). The radial throw reflected at each cutting edge causes the cutting edge trajectory to vary from the ideal trajectory, which is a circle with a diameter equal to the tool diameter. As such, the radial throw at the cutting edges critically affects the attainable dimensional accuracy and surface quality, as well as the micromachining forces. For this reason, accurate determination of effective radial throw of micro-tools is important for practical applications and process-modeling efforts. This paper devises an approach for accurate measurement of radial throw at the cutting edges of micro-tools when using UHS spindles. The approach involves (1) a mathematical framework to determine the trajectory of the cutting edges from measurements of the radial throw at two locations on the tool shank, and (2) an experimental method, based on non-contact displacement measurements, for simultaneous radial throw measurements on the tool shank at two mutually-perpendicular radial directions. The approach is experimentally validated by comparing the predicted and measured radial throw at the tool tip using a micro-tool blank rotated on an UHS spindle at 120,000 rpm. The results indicate that the deviations between the predicted and measured radial throw magnitude and orientation are less than 1% and 0.1 deg., respectively. Subsequently, the validated approach is used to determine tool-tip radial throw of a commercially-available micro-endmill rotated on the same spindle at different spindle speeds. It is concluded that the presented approach provides an effective means for accurate determination of the speed-dependent radial throw at the cutting edges of micro tools towards enhancing the process performance.

Computer Vision-Based Structural Displacement Measurement Robust to Light-Induced Image Degradation for In-Service Bridges

The displacement responses of a civil engineering structure can provide important information regarding structural behaviors that help in assessing safety and serviceability. A displacement measurement using conventional devices, such as the linear variable differential transformer (LVDT), is challenging owing to issues related to inconvenient sensor installation that often requires additional temporary structures. A promising alternative is offered by computer vision, which typically provides a low-cost and non-contact displacement measurement that converts the movement of an object, mostly an attached marker, in the captured images into structural displacement. However, there is limited research on addressing light-induced measurement error caused by the inevitable sunlight in field-testing conditions. This study presents a computer vision-based displacement measurement approach tailored to a field-testing environment with enhanced robustness to strong sunlight. An image-processing algorithm with an adaptive region-of-interest (ROI) is proposed to reliably determine a marker’s location even when the marker is indistinct due to unfavorable light. The performance of the proposed system is experimentally validated in both laboratory-scale and field experiments.

Identification of structural stiffness and excitation forces in time domain using noncontact vision-based displacement measurement

The emerging noncontact vision-based displacement sensor system offers a promising alternative to the conventional sensors for quantitative structural integrity assessment. Significant advantages of the noncontact vision-based sensor include its low cost, ease of operation, and flexibility to extract structural displacement responses at multiple points. This study aims to link the measured displacement data to the quantification of the structural health condition, by validating the feasibility of simultaneous identification of structural stiffness and unknown excitation forces in time domain using output-only vision-based displacement measurement. Numerical analysis are first carried out to investigate the accuracy, convergence and robustness of identified results to different noise levels, sensor numbers, and initial estimates of structural parameters. Then, experiment on a laboratory scaled beam structure is conducted. Results show that the global stiffness of the beam specimen as well as external hammer excitation forces can be successfully and accurately identified from displacement measurement at two points using one camera. The proposed output-only time-domain identification procedure utilizing vision-based displacement measurement represents a low-cost method for either periodic or long-term bridge performance assessment.

Vision-based stress estimation model for steel frame structures with rigid links

This paper presents a stress estimation model for the safety evaluation of steel frame structures with rigid links using a vision-based monitoring system. In this model, the deformed shape of a structure under external loads is estimated via displacements measured by a motion capture system (MCS), which is a non-contact displacement measurement device. During the estimation of the deformed shape, the effective lengths of the rigid link ranges in the frame structure are identified. The radius of the curvature of the structural member to be monitored is calculated using the estimated deformed shape and is employed to estimate stress. Using MCS in the presented model, the safety of a structure can be assessed gauge-freely. In addition, because the stress is directly extracted from the radius of the curvature obtained from the measured deformed shape, information on the loadings and boundary conditions of the structure are not required. Furthermore, the model, which includes the identification of the effective lengths of the rigid links, can consider the influences of the stiffness of the connection and support on the deformation in the stress estimation. To verify the applicability of the presented model, static loading tests for a steel frame specimen were conducted. By comparing the stress estimated by the model with the measured stress, the validity of the model was confirmed.

Digital image correlation-aided mechanical characterization of the anteromedial and posterolateral bundles of the anterior cruciate ligament.

The anterior cruciate ligament (ACL), a major stabilizing ligament of the articular knee joint, is one of the most commonly injured soft tissue structures in the knee. Often, invasive surgery is required to restore pre-injury knee kinematics, and there are several long-term consequences of ACL reconstructions, including early-onset osteoarthritis. The role of the ACL in knee stability and motion has received much attention in the biomechanics community. This paper examines the use of a non-contact full-field displacement measurement technique, digital image correlation, to determine the tensile mechanical properties of the ACL. The focus of this work is to investigate the intrinsic mechanical properties of the ACL, as new knowledge in these areas will aid clinicians in selecting ACL replacements.

Electrostrictive optical resonators for non-contact displacement measurement.

This paper describes a non-contact transduction mechanism for the measurement of linear displacements that is based on the electrostrictive properties of a polymeric optical resonator. The spherical resonators, with a diameter of ∼1 mm and an average optical quality factor of ∼106, are made using a commercially available polymer (Super Soft Plastic-Manufacturing Company). The spherical resonator is immersed in a homogeneous electric field that is generated by applying a voltage difference between two metallic plates. One of the plates is fixed, whereas the other one is movable. By changing the distance between the plates, the electric field intensity changes, leading to a variation of the mechanical forces (electrostrictive effect) acting on the resonator. This effect, in turn, leads to a change in the morphology of the optical resonator and therefore to a shift of its optical resonances. By tracking the shift of the optical modes, it is possible to determine the displacement of the movable plate. Our results indicate a sensitivity ranging from 0.008 to 0.642 pm/μm with a resolution on the order of a few hundreds of nanometers.