3D deformation and shape measurement using phase-stepping DSPI

A new optical arrangement of phase-stepping digital speckle pattern interferom-etry is presented.The system can be used to measure 3D deformation and shape of a curvesurface simultaneously.The phase at each pixel can be determined by appling phase-steppingtechniques.This not only provides high accurate measurement results,but also permits auto-matic analysis of the experimental data.A speckled object reference beam is used,and itmakes the system has the advantage that the measurements are less sensitive to vibration andwhole body motion.     

A stereoscopic digital speckle photography system for 3-D displacement field measurements

Stereoscopic digital speckle photography offers a technique to measure object shapes and 3-D displacement fields in experimental mechanics. The system measures the displacement of a random white light speckle pattern, which somehow is present on the object surface, using digital correlation. This paper describes a general physical model for stereo imaging systems. A camera calibration algorithm, which takes the distortion in the lenses into account, is also presented and evaluated by real experiments. Standard deviations of small deformations as low as 1% of the pixel size for in-plane deformations and 6% of the pixel size for the out-of-plane component are reported. Using the calibration algorithm described, the main source of errors is random errors originating from the correlation algorithm.     

Microscopic 3-D displacement field measurements using digital speckle photography

A technique to measure object shape and 3-D displacement fields in micro-scale is offered by microscopic stereo digital speckle photography. The displacement of the random features that are often present on many engineering surfaces when viewed in a microscope is measured with the system, using image correlation. In this paper the equipment, physical model and calibration routines are described. The technique can be applied for sub-mm sized objects of arbitrary shape for small deformation fields. As a verifying experiment, an in-plane rotation of a flat calibration plate is presented. The expected in-plane errors are shown to be less than 0.1 μm and the corresponding out-of-plane errors about three times larger. As a pilot experiment, micro-structural paper expansion is studied, when exposed to humidity. The scaling properties of the microscope as well as the sampling criteria and reliability of the system are discussed in detail.     

地震振动台实验三维全场位移测量的研究

提出一种基于高速相机双目立体视觉技术的大视场全场三维位移测量方法,用来测量地震振动台实验过程中的位移变化。给出了一种鲁棒的标志点匹配算法,基于VS2010开发环境,研发了用于振动台实验三维全场位移测量系统,设计了精度评估实验方案,验证该方法在大幅面位移测量中的精度,并利用该实验系统对高边坡模型振动台实验进行测量。结果表明:在3 m1.5 m视场范围,静态位移测量误差优于0.4 mm,动态位移测量误差优于0.5 mm,可以满足振动台实验的要求;该方法可以方便、直观地测量地震振动台实验中高边坡模型的位移场,并且测量得到X、Y、Z 3个方向位移曲线以及总位移曲线过渡自然、数据合理,是测量振动台实验全场位移变化的一种有效方法。     

Photothermal modulation of laser diode wavelength: application to sinusoidal phase-modulating interferometer for displacement measurements

In this paper, a novel laser-diode (LD) sinusoidal phase-modulating (SPM) interferometer, which utilizes a photothermal technique for LD wavelength modulation, is proposed to measure displacements with a nanometer accuracy. In conventional LD–SPM interferometers, the LD intensity modulation is concurrent with the wavelength modulation, which increases measurement errors. Using the photothermal technique, the LD wavelength modulation can be accomplished with negligible concomitant intensity modulation, and the measurement errors are thus eliminated. The computer simulations and experiment results verify the usefulness of this novel interferometer.     

Stray field measurements of flow displacement distributions without pulsed field gradients

The probability distribution P(zeta) of diffusive and advective molecular displacements is determined using a fixed field gradient (FFG) pulse sequence, on fluid flow through a Bentheimer sandstone, in the grossly inhomogeneous stray field of a super-conducting magnet. Two decades of q-space are scanned with stimulated echoes, using the gradient of the stray field and variable encoding times delta. The strength of the gradient permits the use of short encoding times, which is desirable for limiting the distorting effects produced by flow displacements through susceptibility induced field inhomogeneities. CPMG and CP echo trains are used to refocus separately the real and imaginary parts of the stimulated echo, for experimental efficiency.     

Medium characterization from interface-wave impedance and ellipticity using simultaneous displacement and pressure measurements

The interface-wave impedance and ellipticity are wave attributes that interrelate the full waveforms as observed in different components. For each of the fluid/elastic-solid interface waves, i.e., the pseudo-Rayleigh (pR) and Stoneley (St) waves, the impedance and ellipticity are found to have different functional dependencies on Young's modulus and Poisson's ratio. By combining the attributes in a cost function, unique and stable estimates of these parameters can be obtained, particularly when using the St wave. In a validation experiment, the impedance of the laser-excited pR wave is successfully extracted from simultaneous measurements of the normal particle displacement and the fluid pressure at a water/aluminum interface. The displacement is measured using a laser Doppler vibrometer (LDV) and the pressure with a needle hydrophone. Any LDV measurement is perturbed by refractive-index changes along the LDV beam once acoustic waves interfere with the beam. Using a model that accounts for these perturbations, an impedance decrease of 28% with respect to the plane wave impedance of the pR wave is predicted for the water/aluminum configuration. Although this deviation is different for the experimentally extracted impedance, there is excellent agreement between the observed and predicted pR waveforms in both the particle displacement and fluid pressure.     

Digital image correlation using iterative least squares and pointwise least squares for displacement field and strain field measurements

Digital image correlation (DIC) method using iterative least squares algorithm (ILS) for displacement field measurement and pointwise least squares algorithm (PLS) for strain field measurement is proposed in this paper. A more general and practical intensity change model is employed with consideration of the linear intensity change of the deformed image, followed by an iterative least squares algorithm for calculating displacement field with sub-pixel accuracy. The concept of correlation function is not used in the ILS method, even though we prove that the algorithm is actually equivalent to the optimization of the sum of squared difference correlation function using improved Newton–Raphson method. Besides, different from the conventional strain estimation approaches based on smoothing the displacement fields first and followed by differentiation of the smoothed displacement fields, a simple yet effective PLS algorithm is proposed for extracting strain fields from the computed displacement fields. The effectiveness and accuracy of the proposed techniques is verified through numerical simulation experiments. A practical application of the algorithms to residual plastic deformation field measurement of GH4169 alloy subjected to tensile fatigue is also presented.     

3D displacement field measurement with correlation based on the micro-geometrical surface texture

Image correlation methods are widely used in experimental mechanics to obtain displacement field measurements. Currently, these methods are applied using digital images of the initial and deformed surfaces sprayed with black or white paint. Speckle patterns are then captured and the correlation is performed with a high degree of accuracy to an order of 0.01 pixels. In 3D, however, stereo-correlation leads to a lower degree of accuracy. Correlation techniques are based on the search for a sub-image (or pattern) displacement field. The work presented in this paper introduces a new correlation-based approach for 3D displacement field measurement that uses an additional 3D laser scanner and a CMM (Coordinate Measurement Machine). Unlike most existing methods that require the presence of markers on the observed object (such as black speckle, grids or random patterns), this approach relies solely on micro-geometrical surface textures such as waviness, roughness and aperiodic random defects. The latter are assumed to remain sufficiently small thus providing an adequate estimate of the particle displacement. The proposed approach can be used in a wide range of applications such as sheet metal forming with large strains. The method proceeds by first obtaining cloud points using the 3D laser scanner mounted on a CMM. These points are used to create 2D maps that are then correlated. In this respect, various criteria have been investigated for creating maps consisting of patterns, which facilitate the correlation procedure. Once the maps are created, the correlation between both configurations (initial and moved) is carried out using traditional methods developed for field measurements. Measurement validation was conducted using experiments in 2D and 3D with good results for rigid displacements in 2D, 3D and 2D rotations.     

Improved 3D displacement measurements method and calibration of a combined fringe projection and 2D-DIC system

An improved measurement method and an automatic calibration procedure are proposed for a combined 2D Digital Image Correlation and Fringe Projection system that allows measuring in- and out-of-plane displacement maps with only one image at each deformation stage of a specimen. The proposed method increases the accuracy and range of the out-of-plane displacements by taking into account the divergences of both the projected fringes (uncollimated) and the camera (with non-zero FOV). The calibration is performed automatically by acquiring a sequence of images of a reference plane by displacing perpendicular to it the camera and fringe projector with a motorized translation stage. The acquired images are then used to obtain a fringe function for each pixel and the necessary parameters required for the correction of the in-plane displacements. Furthermore, a closed form expression is obtained that relates the out-of-plane displacements with the shifted phase at each pixel for a given experimental set-up. This expression is in good agreement with the fringe function obtained by fitting a simple 2nd order polynomial to the experimental obtained calibration data. Finally, the polynomial approach is proposed as a fringe function because it avoids the errors in the determination of the required parameters of the theoretical expression as well as some small misalignment or aberration effects.     

二维及三维流场的光学测量方法

对于复杂的非定常流动 ,流场的测量往往要求无干扰、非接触 ,并且能够瞬时记录流场的二维甚至三维信息。对近年来流场测量领域发展快速、应用广泛的几种光学测量方法 ,如 PIV技术及其由此发展而来的 DPIV和 HPV技术 ,做一些介绍和比较。     

Deriving forces from 2D velocity field measurements

We discuss how to derive a force or a force density from a measured velocity field. The first part focuses on the integral force a fluid exerts on a body, e.g. lift and drag on an airfoil. Obtaining the correct pressure is crucial; however, it cannot be measured within the flow non-intrusively. Using numerical and experimental test cases, we compare the accuracy achievable with three methods: pressure reconstruction from velocity fields via (1) the differential momentum equation, or (2) the Poisson equation, furthermore, (3) Noca’s momentum equation [Noca, JFS 13(5), 1999], which does not require pressure explicitly. The latter gives the best results for the lift, whereas the first or second approach should be used for the drag. The second part deals with obtaining the distribution of a body force density generated by an actuator. Using a stream function ansatz, we obtain a Laplace equation that allows us to compute the solenoidal part of the force distribution; however, the irrotational part is lost. Furthermore, the wall pressure must be known. We validate this approach using numerical data from a wall jet flow in a rectangular box, driven by a fictitious, solenoidal body force. Reconstructing the force distribution yields an error of less than 10^?2 for most of the domain.     

Dynamic two-frequency interferometry for small displacement measurements

Two-frequency interferometry for small displacement measurements is examined from a practical point of view. A laser beam that is modulated sinusoidally in intensity by an acousto-optic Bragg cell provides signal and reference optical waves to produce dynamic interference. The output signal of a photodetector receiving the interference is composed of optical homodyne and heterodyne components. The heterodyne component carries a vibrational displacement to be detected on its amplitude. A sinusoidal vibration amplitude is measured down to 0.5 nm.     

Hybrid fiber interferometer for simultaneous measurement of displacement and temperature

A hybrid fiber interferometer sensing configuration for displacement and temperature measurements is proposed and experimentally demonstrated that is constructed by splicing a short section of polarization maintaining optical fiber to an end-cleaved single mode optical fiber with a tapering structure. The reflected spectrum changes with the variation of displacement and temperature. The sensing configuration uses the method of wavelength and intensity modulations for displacement and temperature measurements, respectively, to which the sensitivities are 0.01392 nm/μm, 0.0214 d Bm/μm,-0.09136 nm∕°C, and 0.15795 d Bm/°C. Experimental results show that displacement and temperature can be measured simultaneously by demodulating the reflected spectrum.     

Automatic recording transmissometer for atmospheric attenuation measurements at 0.9 μm wavelength

The design and performance of a double-ended transmissometer that accurately records the frequency and magnitude of atmospheric attenuation due to mists and fogs is described. Transmission data, covering a 50 dB dynamic range, is recorded automatically at five minute intervals on magnetic tape. The system is battery operated and capable of unattended operation for a period of one month. Results obtained, following the use of the equipment at an outdoor site for a year, are also presented.     

All-optical simultaneous drop and wavelength conversion of DPSK data

We demonstrate an all-optical scheme for the simultaneous drop and wavelength conversion of bursts of data from a continuous stream of differential phase-shift keyed (DPSK) signals. This function is obtained in a single semiconductor optical amplifier Mach-Zehnder interferometer thanks to proper nonlinear interaction of the data stream and an optical gate signal at different wavelength. Fast switching-time enabling wavelength shifting operation on continuous DPSK data stream at 10 and 40 Gb/s without any bit loss is reported. Corresponding measured power penalties are negligible at 10 Gb/s and about 1.7 dB at 40 Gb/s.     

Fiber Bragg grating sensor for simultaneous measurement of displacement and temperature

A new approach to measuring displacement and temperature simultaneously by use of a specially designed isosceles triangular cantilevered beam as a strain agent is demonstrated. A fiber Bragg grating epoxied onto the beam surface is experimentally demonstrated to have a temperature sensitivity of ~0.113 nm/ degrees C below 60 degrees C and a displacement sensitivity of 9.24x10(-2) nm/mm .     

Laser-induced spark for simultaneous ignition and fuel-to-air ratio measurements

This work investigates the use of laser-induced gas breakdown for simultaneously igniting and measuring fuel-to-air ratio of CH₄–air and H₂–air combustible mixtures. The fuel-to-air ratio is determined using the measured spectral peak ratio I_o,Hα/I_o,OI. Sparks are produced using a single-mode, Q-switched Nd–YAG laser. The laser produces a beam of 6 mm in diameter at the wavelength of 1064 nm and pulse duration of 5.5 ns. The beam optics is designed to have mainly a beam splitter and a focusing lens. The beam splitter is coated to reflect the laser beam and transmit emission lines with wavelengths from 600 to 900 nm which are then collected by a fiberoptic cable and detected by an imaging spectrometer–detector assembly. The results showed a linear dependence of the spectral peak ratio on the equivalence ratio that can be generally expressed by φ=a(I_o,Hα/I_o,OI)+b, where a and b are the parameters that depend on the gas pressure. Using the least-square curve fitting technique to fit the experimental data, a calibration curve for calculating the equivalence ratio as a function of the ratio of (I_o,Hα/I_o,OI) was generated.     

Eddy-current non-inertial displacement sensing for underwater infrasound measurements

A non-inertial sensing approach for an Acoustic Vector Sensor (AVS), which utilizes eddy-current displacement sensors and operates well at Ultra-Low Frequencies (ULF), is described here. In the past, most ULF measurements (from mHertz to approximately 10 Hertz) have been conducted using heavy geophones or seismometers that must be installed on the seafloor; these sensors are not suitable for water column measurements. Currently, there are no readily available compact and affordable underwater AVS that operate within this frequency region. Test results have confirmed the validity of the proposed eddy-current AVS design and have demonstrated high acoustic sensitivity.     

A simultaneous in- and out-of-plane displacement measurement method

A method is presented that allows real-time three-dimensional displacement maps to be obtained for deformable objects using a single color camera and a color fringe projector. The acquired images have speckle and fringe information embedded in them, which are separated using the R, G, and B color signals from a color CCD camera to permit the independent use of a two-dimensional digital image correlation to obtain the x- and y-direction displacements and fringe projection to obtain the displacement in the z direction.