Displacement Measurement System Research Articles

Multi-point displacement monitoring of bridges using a vision-based approach

To overcome the drawbacks of the traditional contact-type sensor for structural displacement measurement, the vision-based technology with the aid of the digital image processing algorithm has received increasing concerns from the community of structural health monitoring (SHM). The advanced vision-based system has been widely used to measure the structural displacement of civil engineering structures due to its overwhelming merits of non-contact, long-distance, and high-resolution. However, seldom currently-available vision-based systems are capable of realizing the synchronous structural displacement measurement for multiple points on the investigated structure. In this paper, the method for vision-based multi-point structural displacement measurement is presented. A series of moving loading experiments on a scale arch bridge model are carried out to validate the accuracy and reliability of the vision-based system for multi-point structural displacement measurement. The structural displacements of five points on the bridge deck are measured by the vision-based system and compared with those obtained by the linear variable differential transformer (LVDT). The comparative study demonstrates that the vision-based system is deemed to be an effective and reliable means for multi-point structural displacement measurement.

Structural observation of long-span suspension bridges for safety assessment: implementation of an optical displacement measurement system

This paper addresses the implementation of an optical displacement measurement system in the observation scenario of a long-span suspension bridge and its contribution for structural safety assessment. The metrological background required for quality assurance of the measurements is described, namely, the system's intrinsic parameterization and integration in the SI dimensional traceability chain by calibration, including its measurement uncertainty assessment.

"임의의 위치에서 사용가능한 영상 기반 변위 계측 시스템"의 정오표

"임의의 위치에서 사용가능한 영상 기반 변위 계측 시스템"의 정오표

Compact grating displacement measurement system with a 3 \times 3 coupler

We present a compact displacement measurement system possessing the capability of nanometer-scale precision. On basis of integrating single grating with 3×3 coupler for phase shift in interference signal, the present scheme features advantages of simple structure, convenient alignment, and insensitivity to air turbulence. Linear comparisons between our system and HP5530 show a residual error less than 81 nm during step motions along a 10 mm round-trip, and a discrepancy less than 15 nm in the case of 200 ?m movement. We also demonstrate a measurement stability test in a duration of 300 s, which shows the proposed scheme potentially performs better than HP5530 in terms of long-term stability.

Measurement of the Dynamic Displacements of Railway Bridges Using Video Technology

This article describes the development of a non-contact dynamic displacement measurement system for railway bridges based on video technology. The system, consisting of a high speed video camera, an optical lens, lighting lamps and a precision target, can perform measurements with high precision for distances from the camera to the target up to 25 m, with acquisition frame rates ranging from 64 fps to 500 fps, and be articulated with other measurement systems, which promotes its integration in structural health monitoring systems. The system’s performance was evaluated based on two tests, one in the laboratory and other on the field. The laboratory test evaluated the performance of the system in measuring the displacement of a steel beam, subjected to a point load applied dynamically, for distances from the camera to the target between 3 m and 15 m. The field test allowed evaluating the system’s performance in the dynamic measurement of the displacement of a point on the deck of a railway bridge, induced by passing trains at speeds between 160 km/h and 180 km/h, for distances from the camera to the target up to 25 m. The results of both tests show a very good agreement between the displacement measurement obtained with the video system and with a LVDT.

A High Resolution Displacement Measurement System Based on Time-to-Digital Converter

A microwave interferometry displacement measurement system based on time-to-digital converter (TDC) is proposed. As the TDC has advantages of high time resolution and linearity, it is introduced to replace common phase detector of microwave interferometer. The system is operating at 2.4 GHz in two-stage heterodyne, and its theoretically estimated displacement resolution is about 3 μm. Under the experimental condition of 50 μm moving step, measured maximum error is 30 μm with the standard deviation of 10.9 μm and the linearity is 0.9997.

임의의 위치에서 사용 가능한 영상 기반 변위 계측 시스템

In this study, a vision-based displacement measurement system is developed to accurately measure the displacement of a structure with locating the camera at arbitrary position. The previous vision-based system brings error when the optical axis of a camera has an angle with the measured structure, which limits the applicability at large structures. The developed system measures displacement by processing the images of a target plate that is attached on the measured position of a structure. To measure displacement regardless of the angle between the optical axis of the camera and the target plate, planar homography is employed to match two planes in image and world coordinate systems. To validate the performance of the present system, a laboratory test is carried out using a small 2-story shear building model. The result shows that the present system measures accurate displacement of the structure even with a camera significantly angled with the target plate.

Self-mixing interferometry based on all phase FFT for high-precision displacement measurement

Self-mixing interferometry is an extreme simple and compact laser technique to develop non-contact displacement sensors. In order to enhance the precision of this type of sensor, an improved FFT algorithm, based on all phase FFT instead of conventional FFT, has been proposed to provide a phase reference for self-mixing displacement measurement system which has only single light path and lacks reference. The principle and the experimental realization of the proposed algorithm have been presented. The operation mechanism utilizes all phase FFT, which can estimate the phase from signal accurately, to achieve phase reference when the external target is held stationary and then compensate this reference to the phase when the target starts to move. Resultant displacement value reflects the actual position of the target. Reconstruction errors are on the order of nanometers in the displacements ranging from 100nm to 1μm. Therefore, the proposed method can find application in displacement measurement in industrial and scientific environments.

Elimination of the measurement error induced by piezoelectric ceramic nonlinearity in phase retardation measurement

We introduce a displacement measurement system based on the isotropic optical feedback technique to measure length variations in the feedback cavity. The principle of the system is based on frequency shifting and heterodyne phase-measuring. Using this system, displacements of some feature points in the intensity output of the laser such as polarization switching are measured. Based on the measured results, a linear relation is established between the position of the polarization switching and the anisotropy. This forms the basis of future applications of the polarization control technique in the field of optical measurement.

영상 기반 변위 계측장치의 현장 적용 성능 평가

The on-site performance of a vision-based displacement measurement system (VDMS) was evaluated through a field test on a bridge. The VDMS used in this study is composed of a camera, a marker, a frame grabber, and a laptop. The system measures the displacement by attaching a marker at the location to be measured on the structure, by capturing images of that marker with a fixed rate, and by processing a series of images using a planar homography technique. The developed system was first validated from a laboratory test using a small-scale building structure. The VDMS was then employed in a field test on a railroad bridge with a KTX train running under various conditions. The on-site performance was evaluated by comparing the obtained displacement using the VDMS with the displacement measured from a laser Doppler vibrometer (LDV), which is an expensive and accurate displacement measurement device.

Thermal Borehole Shear Device

This paper presents the details of a new modified borehole shear device that has the capability of measuring the impact of temperature on the in situ shear stress–displacement curves for soil–concrete interfaces. The thermal borehole shear device incorporates concrete shoes with embedded heaters, a pneumatic loading device for application of horizontal normal stresses, and an automated loading system with local vertical displacement and load measurement systems that permits either displacement–control or load–control testing. A methodology for measurement of the soil–concrete shear stress–displacement curves and for evaluation of the drained interface shear strength failure envelopes at different temperatures is presented in this study. Typical results from proof-of-concept tests performed in a clay layer compacted in a laboratory tank in a borehole in a silty sand deposit in the field are presented in this paper. The results are synthesized to show how the impacts of temperature and normal stress on the normalized shear stress–displacement curves can be evaluated. These normalized curves can be measured on a site-specific basis for the calibration of thermo-mechanical load transfer analyses or finite element analyses, which are often used to design and evaluate soil–structure interaction in drilled shaft foundations with geothermal heat exchangers (energy foundations).

Sinusoidal phase-modulating fiber-optic interferometer fringe with a feedback control system.

A displacement measurement system using a fiber-optic interferometer fringe projector with a feedback control system is presented and demonstrated. The system utilizes the integrating bucket method to detect the desired phase or the displacement and Fresnel reflection signal to realize measurement of the disturbance and feed it back to the modulated signal of the laser at the same time. The continuous signal truly reflects the error information, as the output light and reflected light share the same optical path. Practical experiments validate the feasibility of this method.

Novel design and sensitivity analysis of displacement measurement system utilizing knife edge diffraction for nanopositioning stages.

This paper presents a novel design and sensitivity analysis of a knife edge-based optical displacement sensor that can be embedded with nanopositioning stages. The measurement system consists of a laser, two knife edge locations, two photodetectors, and axillary optics components in a simple configuration. The knife edge is installed on the stage parallel to its moving direction and two separated laser beams are incident on knife edges. While the stage is in motion, the direct transverse and diffracted light at each knife edge is superposed producing interference at the detector. The interference is measured with two photodetectors in a differential amplification configuration. The performance of the proposed sensor was mathematically modeled, and the effect of the optical and mechanical parameters, wavelength, beam diameter, distances from laser to knife edge to photodetector, and knife edge topography, on sensor outputs was investigated to obtain a novel analytical method to predict linearity and sensitivity. From the model, all parameters except for the beam diameter have a significant influence on measurement range and sensitivity of the proposed sensing system. To validate the model, two types of knife edges with different edge topography were used for the experiment. By utilizing a shorter wavelength, smaller sensor distance and higher edge quality increased measurement sensitivity can be obtained. The model was experimentally validated and the results showed a good agreement with the theoretically estimated results. This sensor is expected to be easily implemented into nanopositioning stage applications at a low cost and mathematical model introduced here can be used for design and performance estimation of the knife edge-based sensor as a tool.

A versatile stereo photogrammetry based technique for measuring fracture mode displacements in structures

The measurement of fracture mode displacements in structures which are susceptible to cracking such as adhesive joints in composite components – is becoming increasingly important. Such measurements are essential for the understanding of the root causes for specific fracture damage types. Furthermore, they can be used to assess the remaining life span of a structure for its safe operation. An improved version of a previously devised small displacement measurement system (SDMS) is used to measure local relative displacements (LRDs) at the trailing edge of a wind turbine blade. A purpose-made automated image processing software (AIPS) allows a rapid and reliable evaluation of a multitude of subsequently taken measurements at a high-precision level. The SDMS is used to measure the LRDs at three different locations close to the trailing edge of a wind turbine rotor blade. In addition, complementary measurements obtained by linear transducers are compared with the associated LRD component obtained by the SDMS. The 3D LRD measurements showed to be in good agreement with the predictions of non-linear finite element analysis. The paper closes with a brief discussion of the proposed measurement approach and the nature of LRDs as they appear in close vicinity to trailing edge joints.

A Novel Optical Fiber Displacement Measurement System

In order to meet the displacement measurement’s need for high precision and large measuring range, a novel design of optical fiber displacement system is proposed. Optical fiber is applied to the displacement measurement field with its simple structure, light weight, cheap cost and high precision. A displacement measurement model based on transmissive optical fiber is established, and theoretical calculation and simulation are conducted to verify the accuracy of the model. Displacement measuring experiments are completed and the results show that the transmissive optical fiber displacement measurement system has high measuring accuracy with the relative error less than 0.2% over a large range.

Vision-based remote 6-DOF structural displacement monitoring system using a unique marker

Structural displacement is an important indicator for assessing structural safety. For structural displacement monitoring, vision-based displacement measurement systems have been widely developed; however, most systems estimate only 1 or 2-DOF translational displacement. To monitor the 6-DOF structural displacement with high accuracy, a vision-based displacement measurement system with a uniquely designed marker is proposed in this paper. The system is composed of a uniquely designed marker and a camera with a zooming capability, and relative translational and rotational displacement between the marker and the camera is estimated by finding a homography transformation. The novel marker is designed to make the system robust to measurement noise based on a sensitivity analysis of the conventional marker and it has been verified through Monte Carlo simulation results. The performance of the displacement estimation has been verified through two kinds of experimental tests; using a shaking table and a motorized stage. The results show that the system estimates the structural 6-DOF displacement, especially the translational displacement in Z-axis, with high accuracy in real time and is robust to measurement noise.

Non-contact measurement of the dynamic displacement of railway bridges using an advanced video-based system

This article describes the development of a non-contact dynamic displacement measurement system for railway bridges based on video technology. The system, consisting of a high speed video camera, an optical lens, lighting lamps and a precision target, can perform measurements with acquisition frame rates ranging from 64fps to 500fps, and be articulated with other measurement systems, which promotes its integration in structural health monitoring (SHM) systems. Preliminary tests of the system have shown that the measurements’ precision can be affected by: (i) movements of the camera stand and, therefore, rigid supports should be used and the camera should be protected from air flows; (ii) the distortion of the field of view, caused by the flow of heat waves generated by IR incandescent lighting and, therefore, the operating time of the lamps should be limited. The system was used to measure the displacement of a railway bridge’s deck, induced by the passage of trains at speeds between 120km/h and 180km/h, yielding a very good agreement between the results of displacement measurement obtained with the video system and with a LVDT. The achieved precision was below 0.1mm for distances from the camera to the target up to 15m, and in the order of 0.25mm for a distance of 25m. The application of an image processing technique at subpixel level resulted in real precisions generally inferior to the theoretical precisions.

Error Separation and Dynamic Compensation Method in Coplanar Planar Grating Displacement Measurement System

To improve the measurement accuracy of two-dimensional (2D) precision working stages in the semiconductor manufacturing industry, an error separation and dynamic compensation method based on a planar grating is proposed for a 2D coplanar displacement measurement system. In the system, a planar grating of HEIDENHAIN PP281R is fixed in the center of precision working stage to build coplanar structure and reduce Abbe errors. And the system errors including grating period carved error, temperature effect, vibration effect and mount effect are modeled, calculated and separated. Then the separated errors are dynamically compensated from the corresponding displacement measurement results through data-processing software. Analysis results show that the proposed error separation and dynamic compensation method based on planar grating can improve the measurement accuracy of the coplanar displacement measurement system effectively.

A Novel Method to Reduce Accumulative Switching Errors in Multi-Sensor Incremental Measurement Systems

In order to cope with the problems such as nonlinearity and machining difficulty of large gratings or long reflector in the field of long-range and high-precision displacement measurement, multi-sensor incremental measurement systems that combining with switching operations among different sensors has proven to be a feasible solution. However, with large numbers of repetitive switching operations, the switching errors and subsequently generated accumulation become one of the main obstacles for achieving excellent performance. In this paper, a novel method is proposed to reduce the switching errors and their accumulation in the incremental displacement measurement systems. The initialization constant of each “to-be” used sensor is derived by analyzing the real-time measurement data of the adjacent sensors over an extended period and is used for the switching operations. By theoretical analysis and simulation experiments, the proposed method can effectively reduce the accumulative errors relative to conventional methods.

Measurement of local relative displacements in large structures

This article presents a novel measurement technique to measure local relative displacements between parts of large-scale structures. The measured deformations can be of significant importance for fracture analyses in many different types of structures in general and for adhesive connections in particular. The measurement of small local relative displacements in structures subjected to large global deformations is complex and hardly feasible with conventional measurement methods. Therefore, a small displacement measurement system has been devised. The small displacement measurement system is based on stereo photogrammetry and capable of measuring three-dimensional local displacements with a high degree of accuracy. In this article, the technique is used to measure local deformations in the vicinity of the adhesive trailing edge joint of a wind turbine rotor blade. The small displacement measurement system results correspond well with another independent measurement method.