Measurement Of Small Displacements Research Articles

Displacement measurement using an optoelectronic oscillator with an intra-loop Michelson interferometer.

We report on measurement of small displacements with sub-nanometer precision using an optoelectronic oscillator (OEO) with an intra-loop Michelson interferometer. In comparison with conventional homodyne and heterodyne detection methods, where displacement appears as a power change or a phase shift, respectively, in the OEO detection, the displacement produces a shift in the oscillation frequency. In comparison with typical OEO sensors, where the frequency shift is proportional to the OEO oscillation frequency in radio-frequency domain, the frequency shift in our method with an intra-loop interferometer is proportional to an optical frequency. We constructed a hybrid apparatus and compared characteristics of the OEO and heterodyne detection methods.

Comparison of clutter rejection techniques for measurement of small displacements of body surface using radar

To measure vital signs using Doppler radar, a common approach involves the use of time-varying echo phase. To acquire these measurements, clutter rejection is necessary because clutter power is often larger than echo power. To reject static clutter, several techniques have been proposed that assume relatively large phase rotation angles because many studies assume the measurement of the upper torso, where displacement is mainly caused by respiration. However, signals from other parts of the human body are known to have smaller displacements that exhibit small phase rotation angles, which make clutter rejection more difficult. Three clutter rejection techniques for measuring small displacements are compared and their performances are investigated. Using numerical analysis, one method is demonstrated to be the most effective, even for small displacements in noisy data. The best method successfully estimates the centre with an error of −13.4 and −24.0 dB, with a signal-to-noise ratio of 10 and 40 dB and the range of phase rotation angles of 90° and 15°.

Simple optical method for small angular displacement measurement based on the astigmatic effect

We propose a novel small-angle measuring optical method based on the astigmatic effect of two orthogonally placed cylindrical lenses. According to the one-to-one correspondence between the laser spot shape and the angle, the angle is determined. We theoretically analyze the measuring range and demonstrate that a longer distance between the rotating axis and the optical axis leads to a smaller measuring range, but better sensitivity. Also, an associated experimental system is established and a measuring range of 0.94 mrad (cubic fit r=0.9993) as well as a good linear range of 0.37 mrad (linear fit r=0.9994) with a resolution of 8 μrad is achieved.

Modelling of Extrinsic Fiber Optic Sagnac Ultrasound Interferometer Used for Displacement Measurements

Ultrasonic waves are used extensively in nondestructive testing both for characterization of material properties, in this paper, we describe a fiber optic sensor suitable for detection of ultrasonic waves. This sensor is based on an extrinsic fiber optic sagnac interferometer. The proposed sensor model can act as a conventional in-phase detector or as a narrowband detector. In this study we use methods interference of ultrasonic waves between the source of ultrasonic waves and the object under investigation is exploited. The main advantages of the proposed sensor are the ability to detect ultrasonic waves on the surface; this sensor possesses higher sensitivity and accuracy than the pulse method. The cavity resonator was very successfully used for measurement of small ultrasound velocity changes. The ultrasonic interferometric technique based on phase-locked loop is the most suitable for measurements of small displacements. This method ensures the highest sensitivity and accuracy. Keywords : Laser-based ultrasonic, fiber optic sensor, Sagnac interferometer.

Digital ultrasonic speckle phase-shifting method

The digital ultrasonic speckle phase-shifting method (USPM), which is introduced in this paper, can be applied to the measurement of small displacement that is smaller than speckle size at the test point compared to traditional ultrasonic speckle correlation method (USCM). Using USPM, a digital ultrasonic reference signal is introduced to interfere with the ultrasonic speckle signal, which is picked up at the test point on an object surface and is referred to as the object signal. As the phase of the reference signal is shifted several times using the software and then they superimpose with the object signal respectively, the phase of the object signal can be calculated according to the intensities of the superimposed signals. If the object surface moves a small distance, the phase variation of the object speckle can be detected by the same process. As a result, the displacement of the object surface can be measured. Based on the feature of ultrasonic speckles, inner surface displacement of an object can be measured using this proposed method. In this case, the effect of outer surface roughness to the measurement accuracy of USPM is examined experimentally. The experimental results show that the measurement is successful when the displacement is smaller than half of the speckle size at the test point and the roughness parameter Ra of the outer surface of the specimen is less than about 5.47μm.

Effectiveness of observation-domain sidereal filtering for GPS precise point positioning

Sidereal filtering is a technique used to reduce errors caused by multipath in the positioning of static receivers via the Global Positioning System (GPS). It relies upon the receiver and its surrounding environment remaining static from one day to the next and takes advantage of the approximately sidereal repeat time of the GPS constellation geometry. The repeating multipath error can thus be identified, usually in the position domain, and largely removed from the following day. We describe an observation-domain sidereal filter algorithm that operates on undifferenced ionospheric-free GPS carrier phase measurements to reduce errors caused by multipath. It is applied in the context of high-rate (1 Hz) precise point positioning of a static receiver. An observation-domain sidereal filter (ODSF) is able to account for the slightly different repeat times of each GPS satellite, unlike a position-domain sidereal filter (PDSF), and can hence be more effective at reducing high-frequency multipath error. Using 8-h long datasets of GPS measurements from two different receivers with different antenna types and contrasting environments, the ODSF algorithm is shown overall to yield a position time series 5---10 % more stable, in terms of Allan deviation, than a PDSF over nearly all time intervals below about 200 s in length. This may be particularly useful for earthquake and tsunami early warning systems where the accurate measurement of small displacements of the ground over the period of just a few minutes is crucial. However, the sidereal filters are also applied to a third dataset during which two short episodes of particularly high-frequency multipath error were identified. These two periods are analyzed in detail and illustrate the limitations of using sidereal filters with important implications for other methods of correcting for multipath at the observation level.

Small displacement measurements based on an angular-deviation amplifier and interferometric phase detection.

We propose a method for small displacement measurement based on the angle deviation to phase change transformation. The phase change of common-path heterodyne interferometry due to the angle deviation of incidence of a light at interfaces caused by the displacement is detected by a lock-in amplifier. To obtain more accurate results we used an angular amplifier to increase the angle deviation and utilized a surface plasmon resonance (SPR) sensor to enhance the performance of phase detection. When a translator moves one of two face-to-face plane mirrors at an end and then rotates it a small angle, a light is incident onto the mirrors and reflected N times. The outgoing light is also deflected N times of the angle and incident into a SPR sensor. Thus the phase shift due to the angle deviation is amplified N times. The accumulated phase shift is proportional to the amplified angle deviation and displacement. Therefore, the phase change is obtained and the displacement is measured. The amount of movement required can be as low as 0.13 μm without an SPR sensor or 0.08 μm with an SPR sensor. The maximum measurement range can reach 1000 μm.

Spatial profile and singularities of the edge-diffracted beam with a multicharged optical vortex

We analyze the edge diffraction of circular Laguerre–Gaussian beams LG02 and LG03 carrying multicharged optical vortices (OVs), with special attention to spatial properties and evolution of the diffracted beam. The problem is considered numerically within the frame of paraxial approximation and the Fresnel diffraction theory. The diffracted beam evolution is interpreted on the basis of the incident beam symmetry breakdown and decomposition of a higher-order OV into a set of single-charged secondary OVs. Upon propagation, the separate OVs describe specific trajectories within the diffracted beam cross section but in the far field they are localized on the symmetry axis parallel to the screen edge. The OVs' positions in each cross section reflect information of the screen edge position with respect to the incident beam axis. If the incident OV is stopped by the screen, the diffracted beam possesses no OV just behind the screen but in the course of its further propagation, single-order OVs appear consecutively so that in the far field the number of OVs equals to the absolute topological charge of the incident OV. Possible applications for the remote measurements of small linear displacements are discussed.

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.

Experimental Demonstration of a Displacement Measurement of an Optical Beam beyond the Quantum Noise Limit

We experimentally generate a spatially squeezed light beam and realize a small-displacement measurement beyond the quantum noise limit with this squeezed light. Moreover, we measure about −2.2±0.2 dB spatial squeezing and reduce the minimum measurable displacement from 1.17 Å to 0.99 Å with the signal-to-noise ratio normalized to 1.

Small-displacement measurements using high-order Hermite-Gauss modes

We present a scheme for small-displacement measurements using high-order Hermite-Gauss modes and balanced homodyne detection. We demonstrate its use with experimental results of displacement measurements using fundamental transverse mode TEM00 and first order transverse mode TEM10 as signal modes. The results show a factor of 1.41 improvement in measurement precision with the TEM10 mode compared with that with the TEM00 mode. This scheme has potential applications in precision metrology, atomic force microscopy, and optical imaging.

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.

DESIGN AND DEVELOPMENT OF A LINEAR VARIABLE DIFFERENTIAL OPTICAL SENSOR FOR SMALL RANGE LINEAR DISPLACEMENT MEASUREMENT

In this present paper, we have illustrated the deve lopment of a novel linear displacement sensor utili zing the intensity of light. This sensor is based on temporal changes in the intensit y of diffuse light beam for the movement of a solid metallic obstacle between the source and detector instead of traditional reflecto r. An obstacle is moved over open surface of a holl ow shaped channel, where light beams impinged vertically into the channel. Two pho to detectors has been used to sense the variation o f diffuse light intensity at the covered & uncovered area of channel surface due to the movement of obstacle over its open surface. Thi s movement of an obstacle represents the actual target displacement by differ entiating the output transformation of two photo de tectors; this phenomenon is reported as linear variable differential optical se nsor (LVDOS). The measured displacement recorded in terns of voltage by the signal processing circuit (SPC). Experimental results are shown a satisfactory performance of the sensor for small range displacement measurement.

Characteristics of two-wave mixing adaptive interferometer with CdTe:Ge at 106 and 155 μm and improved temporal adaptability with temperature control

We have characterized a two-wave mixing adaptive interferometer based on dc-biased photorefractive CdTe:Ge crystal at 1.06 and 1.55 μm. Excellent performance is shown at both wavelengths by demonstration of high sensitivity for measurement of small displacements and high cutoff frequency at low intensity. We have achieved a considerable reduction of the undesired low-frequency response using controlled heating of the crystal, which ensures depopulation of corresponding traps. The experimental data are used for measurement of the real and imaginary parts of the coupling constant, as well as the dielectric relaxation time of the crystal and the mobility-lifetime product of the free charge carriers.

Small angular displacement measurement based on an autocollimator and a common-path compensation principle

A novel method for small angular displacement measurement based on an autocollimator and a common-path compensation principle by using single CCD detector was proposed. The principles of the angular displacement measurement and the common-path compensation were analyzed. The feasibility of measurement method was verified and the experimental results revealed that the linear correlativity between the relative displacement of the measuring beam spot and the angular displacement is 0.99996. And the measurement resolution is about 0.03 arcsec. To test the compensation's effect, a series of experiments introducing three different interferences from system and external environment were performed. The experimental results indicated that the standard deviations of the measuring beam spot's angular drift were improved by at least 25.0% to at most 80.0% in x direction while by at least 28.2% to at most 95.6% in y direction. Thus, the stability of the system and the measurement resolution were improved.

A three-frame digital image correlation (DIC) method for the measurement of small displacements and strains

Digital image correlation (DIC) has become a well-established approach for the calculation of full-field displacement and strains within the field of experimental mechanics. Since their introduction, DIC methods have been relying on only two images to measure the displacements and strains that materials undergo under load. It can be foreseen that the use of additional image information for the calculus of displacements and strains, although computationally more expensive, can positively impact DIC method accuracy under both ideal and challenging experimental conditions. Such accuracy improvements are especially important when measuring very small deformations, which still constitutes a great challenge: small displacements and strains translate into equally small digital image intensity changes on the material’s surface, which are affected by the digitization processes of the imaging hardware and by other image acquisition effects such as image noise. This paper proposes a new three-frame Newton–Raphson DIC method and evaluates it from the standpoints of accuracy and speed. The method models the deformations that are to be measured under the assumption that the deformation occurs at approximately the same rate between each two consecutive images in the three image sequences that are employed. The aim is to investigate how the use of image data from more than two images impacts accuracy and what is the effect on the computational speed. The proposed method is compared with the classic two-frame Newton–Raphson method in three experiments. Two experiments rely on numerically deformed images that simulate heterogeneous deformations. The third experiment uses images from a real deformation experiment. Results indicate that although it is computationally more demanding, the three-frame method significantly improves displacement and strain accuracy and is less sensitive to image noise.

FPGA-Based Smart Sensor for Online Displacement Measurements Using a Heterodyne Interferometer

The measurement of small displacements on the nanometric scale demands metrological systems of high accuracy and precision. In this context, interferometer-based displacement measurements have become the main tools used for traceable dimensional metrology. The different industrial applications in which small displacement measurements are employed requires the use of online measurements, high speed processes, open architecture control systems, as well as good adaptability to specific process conditions. The main contribution of this work is the development of a smart sensor for large displacement measurement based on phase measurement which achieves high accuracy and resolution, designed to be used with a commercial heterodyne interferometer. The system is based on a low-cost Field Programmable Gate Array (FPGA) allowing the integration of several functions in a single portable device. This system is optimal for high speed applications where online measurement is needed and the reconfigurability feature allows the addition of different modules for error compensation, as might be required by a specific application.

Measurement of small displacement based on surface plasmon resonance heterodyne interferometry

Abstract This paper proposes an optical method for measuring small displacements using the surface plasmon resonance (SPR) heterodyne interferometry. A heterodyne light beam reflected by a mirror passes through a hemisphere glass and then enters into a surface plasmon resonance apparatus at the resonant angle. A small displacement of the mirror will introduce a phase-difference variation between p - and s -polarizations of the light emerging from the SPR apparatus. The phase-difference variation can be precisely measured with the heterodyne interferometric technique, and the associated displacement can be estimated. The feasibility of this method was verified by experiment, and the displacement measurement resolution of about 1.4 nm over a traveling range of 6 μm was achieved. Our method of measurement has the merits of both common-path interferometry and heterodyne interferometry.

Research on the Moire fringes formed by circular and linear grating

The features of Moire fringe generated by overlapping a circular and a linear grating are studied. Given that the pitch of circular grating is a and that of linear grating is P, the shapes of the Moire fringe that they form are hyperbola, parabola, and ellipse when a > P, a = P, and a < P, respectively. As the pitches of these two gratings become close to each other, the magnification of the Moire fringe is over 100, which is useful for the measurement of small displacement. This letter also discusses how the fill factor influences Moire fringe visibility.

Fabry–Perot interference-based fiber-optic sensor for small displacement measurement

A simple fiber-optic sensor based on Fabry–Perot interference for small displacement measurement is investigated both theoretically and experimentally. A broadband light source is coupled into a Fabry–Perot cavity formed by the surfaces of a sensing fiber end and the measured object. The interference signals from the cavity are reflected back into the same fiber. A small displacement via changes in cavity length can be obtained by measuring the wavenumber spacing of the interference fringes. The experimental data meet with the theoretical values very well. Given the light source bandwidth of 40 nm and the initial distance of 30 μm, the proposed displacement sensing system could achieve high resolution measurement of 16 nm.