Electronic Speckle Pattern Interferometry System Research Articles

Simultaneous three-dimensional deformation measurement using tri-wavelength and monochrome camera imaging with a pyramid prism.

What we believe to be a new electronic speckle pattern interferometry (ESPI) configuration is being developed for simultaneous three-dimensional deformation measurements. In this ESPI system, two pairs of symmetrical illuminating arrangement with dual-wavelength lights were used to independently sense two in-plane deformation components, one Michelson interferometer-based set illuminating with the other wavelength light was utilized to measure out-of-plane deformation. The color speckle interferogram was split into four sub-patterns by a prism, three of them were filtered by three different bandpass dichroic filters and recorded by one monochrome camera. Micro-rotation testing work firstly verifies the validation of the proposed phase-shifting device. Three-dimensional deformation information was simultaneous obtained by using temporal phase-shift method. All strain components related to the specimen surface deformation were further determined by numerical differential. The experimental results of a tested specimen were excellently consistent with those of FEM simulation, which verified the validation and feasibility of the proposed ESPI system for measuring 3D deformation.

Accurate determination of the elastic moduli of optimized cantilever beams by efficient time-averaged ESPI system

Elastic moduli, including Young’s modulus, shear modulus, bulk modulus, etc, are key parameters that are used to characterize the ability of a solid material to resist various types of deformation. The moduli can be extracted from the natural frequencies of a cantilever beam. In this paper, the relationships between moduli and natural frequencies, for the first time, are quantified by the finite element method. The optimized three-dimensional proportion of the cantilever beam is selected to be implemented simple error compensation. Experimentally, to precisely obtain the natural frequencies of the cantilever beam, an efficient time-averaged electronic speckle pattern interferometry(ESPI) system has been developed. The efficiency and precision are reflected in the following aspects: firstly, according to the slender character of the cantilever beam, a large shear optical path arrangement is designed to facilitate isolation from environmental interference; secondly, a resonance search method, based on the moiré effect is employed to recognize the natural frequencies accurately and efficiently; thirdly, a novel dynamic phase-shifting method is proposed based on the arrangement of the large shear optical path for clearer visualization of the mode shape of the cantilever beam. The proposed methods are verified by three kinds of common materials. The results suggest that Young’s modulus and shear modulus derived from natural frequencies are higher than the known value, and the error compensation can significantly reduce the calculation error. Furthermore, the experiments carried out on the woven carbon fiber reinforced plastic laminates illustrate the potential of the proposed methods in the evaluation of elastic moduli of composites. Given that the exciter attached to the specimen surfaces can be replaced with some special counterparts, the proposed ESPI system has considerable potential to test the objects loaded in some extreme environments, e.g. at high temperatures or underwater, where contact detection methods are difficult to be implemented.

Panoramic amplitude-fluctuation electronic speckle pattern interferometry system for inner vibration measurement

We propose a panoramic vibration measurement system for the inner cylindrical surface of an object based on amplitude fluctuation electronic speckle pattern interferometry and developed according to the Michelson interference configuration. A conical mirror, aligned to the axis of the inner cylindrical surface, is illuminated to transform a single camera view into a panoramic view. After transposing the observation coordinate system, a complete inner 360-deg vibration pattern can be reconstructed. The principle of transformation from a Cartesian-coordinate system to a cylindrical-coordinate system is introduced in detail. The panoramic vibration of the inner circular tube is determined, and the first six modes are analyzed. The comparison and error analysis of the proposed system with the ANSYS simulation results prove the accuracy and feasibility of the system.

Measurement of natural frequencies and mode shapes of transparent insect wings using common-path ESPI.

In this study, a common-path electronic speckle pattern interferometry system which upholds the natural property of transparency of insect's wings has been developed to measure the wings' natural frequencies and mode shapes for the first time. A novel base-exciting method was designed to enable the simultaneous application of sinusoidal and static forces to excite wings and introduce an additional phase. The moiré effect induced by the amplitude modulation was employed to accurately recognize the resonance state. Subsequently, the mode shapes were visualized by phase-shifting and real-time frame subtraction. Eight pairs of forewings from cicadas were investigated. The first three order natural frequencies of the wings are approximately 145 Hz, 272 Hz and 394 Hz, respectively, which are dispersed to prevent modal coupling. The cambered mode shapes exhibit a strongly spanwise-chordwise anisotropy flexural stiffness distribution, generally dominated by bending and twisting deformation. The details of the high-order mode shapes show that the tip exhibits distinct deformation, indicating more flexibility to cope with external impact load, and the nodal lines usually comply with the direction of the wing veins in higher modes, substantiating the fact that the veins play an important role as stiffeners of the membrane. The results are in excellent agreement with the dynamic performance of previous studies, which will potentially affect a broader community of optical measurement specialists and entomologists to enhance our understanding of time-averaged interferograms and insect flights.

Experimental verification of full-field accuracy in stereo-DIC based on the ESPI method.

This paper proposes a method to merge stereo-digital image correlation (DIC) and electronic speckle pattern interferometry (ESPI) data by camera calibration. The proposed method is employed to verify the accuracy of full-field out-of-plate displacements measured by stereo-DIC in a cantilever beam test. The mean absolute error and the root mean square error (RMSE) of the full-field displacement measured by four-megapixel cameras are 0.849 µm and 1.08 µm at 60 mm field of view, respectively, and the RMSE of the central area is 0.615 µm. The errors are not uniformly distributed because of the imperfect calibration. When the lenses are changed and the field of view reaches 120 mm, the RMSE is 1.48 µm with uniform distribution. These accuracies could be traced back to the laser wavelength to confirm the stereo-DIC data. The proposed method can be used not only to verify the full-field measurement accuracy of DIC but also to determine the rigid-body displacement for ESPI with a high-precision stereo-DIC. Thus, the displacement vector can be obtained. Furthermore, it can unify the coordinate of multiple ESPI systems to achieve a large range of high-precision three-dimensional deformation measurements.

Tri-wavelength simultaneous ESPI for 3D micro-deformation field measurement.

Electronic speckle pattern interferometry (ESPI), a well-established technique for micro-deformation measurement, can be used to determine both in-plane and out-of-plane displacement components. Although many works in ESPI have been reported for three-dimensional (3D) displacement measurement, few works have focused on the simultaneous measurement of 3D deformation fields. Here we present an ESPI system that consists of three sub-interferometers for simultaneous measurement of all three displacement components and in-plane strain fields. A 3CCD color camera, a specially designed shifting stage, and three lasers with optimal wavelengths are used in this system. The lasers and 3CCD camera provide independent interferograms with different color signals, while the shifting stage allows the sub-interferometers to achieve simultaneous phase shifting. The results of color separation and experimental measurement demonstrate the utility of the system.

Review and Comparison of Different Multi-Channel Spatial-Phase Shift Algorithms of Electronic Speckle Pattern Interferometry

<div class="section abstract"><div class="htmlview paragraph">Electronic Speckle Pattern Interferometry (ESPI) is the most sensitive and accurate method for 3D deformation measurement in micro and sub micro-level. ESPI measures deformation by evaluating the phase difference of two recorded speckle interferograms under different loading conditions. By a spatial phase shift technique, ESPI allows for the rapid, accurate and continuous 3D deformation measurement. Multi-channel and carrier frequency are the two main methods of spatial phase shift. Compared with carrier frequency method, which is subject to the problem of spectrum aliasing, multi-channel method is more flexible in use. For extracting the phase value of speckles, four-step algorithm and five-step arbitrary phase algorithm are commonly used. Different algorithms have different spatial resolution, operational requirements, and phase image quality. In order to better understand the impact of different algorithms, we present a comparison of these methods of spatial phase measurement in terms of quality and operability. Besides, this paper also conducts experimental research and discussion on the relationship between speckle and corresponding pixels. Basic theory, brief derivation and optical layouts for the three algorithms will be presented. The potentials and limitations of the multi-channel spatial phase shift ESPI system will be demonstrated by examples of precise measurement of deformation.</div></div>

Constructing an electronic speckle pattern interferometry system for visualizing operational deflection shapes

Electronic speckle pattern interferometry (ESPI) is an effective optical method for visualizing operational deflection shapes of vibrating objects. The time-averaged ESPI method is used to show how an object vibrates when driven sinusoidally. We describe the construction of our system, which we built using standard optical components. Such a system is appropriate for both research and educational purposes. We will discuss the lessons learned by constructing several ESPI systems.

Dynamic parallel phase-shifting electronic speckle pattern interferometer.

Methods for measuring variations in diffuse surfaces using electronic speckle pattern interferometry (ESPI) are widely used and well known. In this research, we present an out-of-plane ESPI system coupled to a Michelson configuration to generate simultaneous parallel interferograms with different phase shifts. The system uses circular polarization states to generate parallel phase shifted interferograms. Due to the polarization states, the fringes do not experience a contrast reduction, thus avoiding measurement errors that affect spatial or temporal phase-shifting in interferometry. The basic operating principle of polarization modulation is described, and results that represent the temporal evolution of an aluminum plate are presented. The generation of two simultaneous patterns allows one to track the dynamic performance of the plate.

Double speckle pattern interferometric measurements for micro-angular displacement and the center of rotation

Double speckle pattern interferometry is presented for the measurement of in-plane rotation angle, sign, and center of rotation. The technique employs two conventional in-plane sensitive electronic speckle pattern interferometry systems combined with two-wavelength laser illumination and a phase-shifting method. Angular displacement of micro-rotation including the sign is determined from the wrapped phase difference, and the center of rotation is located by using wrapped phase difference maps related to two-directional displacement. The test setup is described and experimental results indicate that the system can provide angular displacement measurement with accuracy of 1.8 arcsec.

Temporal electronic speckle pattern interferometry for real-time in-plane rotation analysis.

A temporal electronic speckle pattern interferometry (ESPI) system is proposed for in-plane rotation measurement. The relationship between the rotation angle and the phase change distribution is established and the rotation direction is indicated by the sign of the partial differential of the phase change distribution. Temporal phase modulation is applied in the proposed symmetric illumination ESPI system. The phase is recovered by the temporal intensity analysis method which uses the temporal evolution history of the light intensity. The system can perform dynamic measurements and provide results in off-line real-time. Preliminary experiments were carried out with a continuously rotating target to show the feasibility and the dynamic feature of the temporal ESPI system. At present, the mean absolute error of the experiment is 0.39 arcsec.

Simultaneous 2D in-plane deformation measurement using electronic speckle pattern interferometry with double phase modulations

Electronic speckle pattern interferometry (ESPI) and digital speckle pattern interferometry are well-established non-contact measurement methods. They have been widely used to carry out precise deformation mapping. However, the simultaneous two-dimensional (2D) or three-dimensional (3D) deformation measurements using ESPI with phase shifting usually involve complicated and slow equipment. In this Letter, we solve these issues by proposing a modified ESPI system based on double phase modulations with only one laser and one camera. In-plane normal and shear strains are obtained with good quality. This system can also be developed to measure 3D deformation, and it has the potential to carry out faster measurements with a high-speed camera.

Residual stress measurement on the butt-welded area by electronic speckle pattern interferometry

Residual stress always exists on any kind of welded area. This residual stress can cause the welded material to crack or fracture. For many years, the hole-drilling method has been widely used for measuring residual stress. However, this method is destructive. Nowadays, electronic speckle pattern interferometry (ESPI) can be used to measure residual stress with or without the hole-drilling method. ESPI is an optical nondestructive testing methods that use the speckle effect. Mechanical properties can be measured by calculation of the phase difference by the variation of temperature, pressure, or loading force. In this paper, the residual stress on the butt-welded area is measured by using ESPI with a suggested numerical calculation. Two types of specimens are prepared. Type I is made of pure base metal part and type II has a welded part at the center. These specimens are tensile tested with a material test system. At the same time, the ESPI system was applied to this test. From the results of ESPI, the elastic modulus and the residual stress around the welded area can be calculated and estimated. With this result, it is confirmed that the residual stress on the welded area can be measured with high precision by ESPI.

The design of a non-cube beam-splitter used for electronic speckle pattern interferometry

A non-cube beam-splitter (NCBS) is proposed, by which an incident beam can be separated largely in a direction and then the lights from the test object and the lights from a reference surface placed adjacently to the test object can be combined to construct a simple electronic speckle pattern interferometry (ESPI) system. Two mainly useful quantitative ways, to calculate the phase change of the test object, the phase-shift and the fringe carrier method with Fourier transform, can be achieved in the ESPI system with the NCBS. Experiments with phase-shifting and fringe carrier method are completed. The experimental results show that the monolithic design of the proposed NCBS is effective in ESPI measurement and immunity to vibrations.

Review of electronic speckle pattern interferometry (ESPI) for three dimensional displacement measurement

Three dimensional(3D) displacements, which can be translated further into 3D strain, are key parameters for design, manufacturing and quality control. Using different optical setups, phase-shift methods, and algorithms, several different 3D electronic speckle pattern interferometry(ESPI) systems for displacement and strain measurements have been achieved and commercialized. This paper provides a review of the recent developments in ESPI systems for 3D displacement and strain measurement. After an overview of the fundamentals of ESPI theory, temporal phase-shift, and spatial phase-shift techniques, 3D deformation measurements by the temporal phase-shift ESPI system, which is suited well for static measurement, and by the spatial phase-shift ESPI system, which is particularly useful for dynamic measurement, are discussed. For each method, the basic theory, a brief derivation and different optical layouts are presented. The state of art application, potential and limitation of the ESPI systems are shown and demonstrated.

Optical-fiber Electronic Speckle Pattern Interferometry for Quantitative Measurement of Defects on Aluminum Liners in Composite Pressure Vessels

Optical-fiber electronic speckle pattern interferometry (ESPI) is a non-contact, non-destructive examination technique with the advantages of rapid measurement, high accuracy, and full-field measurement. The optical-fiber ESPI system used in this study was compact and portable with the advantages of easy set-up and signal acquisition. By suitably configuring the optical-fiber ESPI system, producing an image signal in a charge-coupled device camera, and periodically modulating beam phases, we obtained phase information from the speckle pattern using a four-step phase shifting algorithm. Moreover, we compared the actual defect size with that of interference fringes which appeared on a screen after calculating the pixel value according to the distance between the object and the CCD camera. Conventional methods of measuring defects are time-consuming and resource-intensive because the estimated values are relative. However, our simple method could quantitatively estimate the defect length by carrying out numerical analysis for obtaining values on the X-axis in a line profile. The results showed reliable values for average error rates and a decrease in the error rate with increasing defect length or pressure.

Tailoring the performance of flexible electret loudspeakers by varying cell actuator formation

We present a design of electret-based soft cell actuators which can improve the performance of flexible electret loudspeakers. Electret loudspeakers have advantages over traditional speakers such being thin, lightweight and flexible. They have potential application to 4C products (e.g. computers, communication, consumer electronics, and cars). Flexible electret loudspeakers are composed of an array of small cell actuators. The first mode of each electret cell actuator greatly affects the low frequency of an electret loudspeaker. To tailor the bandwidth of an electret loudspeaker for different applications, the shape and size of each cell actuator can be varied according to the specific requirements. A finite element analysis (FEA) model was used to simulate a vibration mode. An AVIDTM (advanced vibrometer/interferometer device) system and ESPI (electronic speckle pattern interferometry) system were two non-destructive optical detection tools used to detect vibration and the corresponding vibration mode shape of the electret cell actuators, respectively. Our experimental results verified the simulated results. We showed that finite element analysis (FEA) can be used to design flexible loudspeakers. We can tailor the sound characteristics of an electret loudspeaker by varying the size and shape of cell actuators such that flexible electret loudspeaker performance can be dramatically improved.

Opening and Mixed-Mode Fracture Initiation in a Quasi-Brittle Material

An electronic speckle pattern interferometry (ESPI) system, which delivers high-resolution displacement data from fringes that are formed by the subtraction of laser speckle patterns, was constructed to study fracture initiation in a quasi-brittle material. Mode I opening and Mixed-Mode I and II fracture experiments were performed with a homogeneous, fine-grained (0.1–0.8 mm grain size) sandstone using the three-point bending test. Specimens were notched at various lengths and positions of the beam edge to produce the desired loading condition, with KII/KI=0-13%. The experimental results indicate that the length of the localized damage zone at peak load for Mode I fracture is 6–7 mm, which is about 10 times the (largest) grain size. From the mixed-mode loading tests, the zone length at peak load increased to 10–12 mm, and the length was more or less constant for KII/KI=5-13%. ESPI also provided detailed information on the horizontal displacement profiles along the damage zone. For center notch specimens at peak load, the horizontal (opening) displacement at the notch tip was 40 μm, which can be interpreted as the critical opening displacement if the damage zone is fully formed at peak. For mixed-mode specimens, the critical horizontal displacement at peak load is 60–80 μm, but the vertical displacement is needed to resolve the critical opening and sliding components.

A simple and inexpensive optical technique to help students visualize mode shapes

An imaging technique is introduced that is suitable for visualizing the mode shapes of vibrating structures in an educational setting. The method produces images similar to those obtained using electronic speckle pattern interferometry (ESPI), but it can be implemented for less than 1/10 the cost of a commercial ESPI system, and the apparatus is simple enough that it can be constructed by undergraduate students. This technique allows for real-time visualization of the normal modes and deflection shapes of harmonically vibrating structures, including those with shapes that make generating Chladni patterns with sand or powder impossible. The theory of operation and construction details are discussed.

Piezoelectric d15 shear response-based torsion actuation mechanism: an experimental benchmark and its 3D finite element simulation

An experimental benchmark is proposed for piezoelectric, direct-torsion actuation using mono-morph piezoceramic d15 shear patches. This is reached by designing and assembling an adaptive plate having two identical composite faces sandwiching a core made of connected six oppositely polarized (OP) piezoceramic d15 shear patches along the length. An electronic speckle pattern interferometry system was used to measure the static tip deflection of the adaptive sandwich composite plate that was mounted in a cantilever configuration and actuated in torsion by progressively applied voltages on the piezoceramic shear core electroded major surfaces. Then, the effective rate of twist was post-processed and proposed as an evaluation criterion for smart composites under piezoelectric torsion actuation. For verification of the experimental results, the proposed experimental benchmark was simulated using three-dimensional piezoelectric finite elements (FE) within ABAQUS® commercial software. The comparison of the obtained experimental and simulation results showed reasonable agreement, but the slight nonlinear experimental response was not confirmed by the linear FE analysis. The experimentally proved torsion actuation mechanism, produced by OP piezoceramic d15 shear patches, can be applied actively to prevent torsion in many applications, such as in wind turbines, helicopter blades, robot arms, flexible space structures, etc.